Short answer: Yes, within limits. Under IRS Notice 2016-16, you can change a safe harbor 401(k) during the plan year as long as the change isn’t on the IRS’s prohibited list and, if it affects the safe harbor notice, employees get an updated notice and a chance to change their contribution. Separately, the SECURE Act lets an existing 401(k) become safe harbor mid-year with a 3% nonelective contribution if you amend at least 30 days before the plan year ends, or 4% if you amend later. Basic Capital, as your 3(16) plan administrator, sends the required safe harbor notices automatically.

The two rules behind every mid-year change

Two separate sets of rules decide what you can do:

1. Notice 2016-16 covers changes to a plan that is already safe harbor. It says a mid-year change doesn’t break safe harbor status just because it happens mid-year, if the notice and election conditions are met and the change isn’t prohibited.

2. Treasury Regulation §1.401(k)-3 and the SECURE Act cover the bigger moves: becoming safe harbor, reducing or suspending safe harbor contributions, and dropping safe harbor status. Notice 2016-16 says these are allowed “only as described in existing regulations.”

So the first question is always: are you tweaking a safe harbor plan, or changing its safe harbor status?

Changes the IRS allows mid-year

The IRS lists these as permissible mid-year changes, provided the notice rules are met where they apply:

• Increasing future nonelective contributions from 3% to 4% for all eligible employees

• Adding an in-service withdrawal at age 59½

• Changing the plan’s default investment fund

• Changing the plan’s rules on arbitration of disputes

• Moving the plan entry date from monthly to quarterly for newly eligible employees

• Adopting amendments required by law, such as a statute change or court decision

These are examples, not a complete list. Any change that isn’t prohibited can work if it follows the notice rules below.

Changes the IRS prohibits mid-year

Notice 2016-16 lists four changes you generally can’t make during the plan year:

The match exception is useful in practice. If business is good and you want to raise the match for 2027, you can, as long as you adopt the change and send the notice at least 3 months before the year ends (the IRS gives no calendar date, so for a calendar-year plan, finish by September 30 to be safe) and apply it to the whole year.

The notice rule for any mid-year change

If a change affects what the safe harbor notice says, you must:

1. Give every employee who gets the safe harbor notice an updated notice describing the change and its effective date, within a reasonable period before it takes effect. Giving it 30 to 90 days before the effective date counts as reasonable.

2. Give each employee a reasonable period to change their contribution election before the change takes effect. A 30-day window counts as reasonable.

If advance notice isn’t practical (for example, a change that is retroactive by design), Notice 2016-16 allows the notice and election opportunity as soon as practical, and no later than 30 days after the change is adopted.

Becoming safe harbor mid-year (SECURE Act nonelective rule)

Since plan years beginning in 2020, an existing 401(k) can switch to a nonelective safe harbor partway through the year (SECURE Act §103):

The SECURE Act also removed the annual safe harbor notice for nonelective plans. Employees must still be able to change their contribution election at least once a year.

This is the main fix for a plan that is headed for a failed ADP or ACP test. Instead of refunding money to highly compensated employees, you can amend to a nonelective safe harbor for that year. The trade-off is cost: 3% or 4% of pay for every eligible employee, including people who don’t defer.

A safe harbor match can’t be added mid-year to an existing plan. It has to be in place, with the notice given, before the plan year begins. The exception is a brand-new plan, covered in safe harbor 401(k) for startups.

Reducing, suspending or dropping safe harbor mid-year

You can reduce or suspend safe harbor contributions during the year, but only under §1.401(k)-3(g). In short:

• Either the business is operating at an economic loss for the year, or the safe harbor notice given before the year said contributions could be reduced or suspended mid-year.

• All eligible employees get a supplemental notice explaining the change.

• The reduction takes effect no earlier than the later of the date the amendment is adopted or 30 days after the supplemental notice.

• Employees get a reasonable chance to change their elections first.

• The plan must pass the ADP test (and ACP test, if it has a match) for the whole year, and you must fund safe harbor contributions earned up to the effective date.

In other words, dropping safe harbor brings back the testing you were avoiding, for the whole year. Plan this with your administrator before sending any notice.

2027 deadlines for a calendar-year plan

Dates are calculated from the cited rules for a January to December plan year. Non-calendar plan years shift every date. For 2027 the IRS set the employee deferral limit at $25,500 (IRS Notice 2026-66).

How Basic Capital handles mid-year safe harbor changes

Basic Capital is the recordkeeper and, through Basic Capital Recordkeeping Services LLC, the 3(16) plan administrator for the plans it serves. For safe harbor plans that means:

• Required participant notices go out automatically 30 to 90 days before each plan year.

• The compliance dashboard tracks ADP, ACP and top-heavy status through the year, so a likely test failure shows up while the 3% nonelective option is still open.

• Basic Capital handles plan administration, compliance testing and the Form 5500 filing.

See how the Basic Capital safe harbor 401(k) works, or compare safe harbor match formulas.

Frequently asked questions

Can I switch from a traditional 401(k) to a safe harbor match in the middle of the year? Not for the current year. A safe harbor match must be in place before the plan year begins. Mid-year, your option is a 3% or 4% nonelective safe harbor under the SECURE Act. You can set up a match for the following year with a notice 30 to 90 days before it starts.

Can I lower our safe harbor match mid-year? Only under §1.401(k)-3(g): you need an economic loss for the year or a notice that warned of possible reductions, a supplemental notice, and at least 30 days’ lead time. The plan then loses safe harbor status for that year and must pass ADP and ACP testing.

Can I raise our safe harbor match mid-year? Yes, if you adopt the change at least 3 months before the plan year ends, make it retroactive to the start of the year and give employees an updated notice and election opportunity in that same window (Notice 2016-16).

Do we need to send a notice for a nonelective safe harbor? Not the annual safe harbor notice, since the SECURE Act removed it for nonelective plans starting in 2020. A mid-year change that affects a notice you did give still follows the Notice 2016-16 rules.

References

• Internal Revenue Service. (2025, November 16). Mid-year changes to safe harbor 401(k) plans and notices. https://www.irs.gov/retirement-plans/mid-year-changes-to-safe-harbor-401k-plans-and-notices

• Internal Revenue Service. (2016). Notice 2016-16, Mid-Year Changes to Safe Harbor Plans and Notices. https://www.irs.gov/pub/irs-drop/n-16-16.pdf

• Internal Revenue Service. (2026, August 4). 401(k) plan overview. https://www.irs.gov/retirement-plans/plan-sponsor/401k-plan-overview

• 26 CFR §1.401(k)-3, Safe harbor requirements (paragraphs (d), (e) and (g)). https://www.ecfr.gov/current/title-26/chapter-I/subchapter-A/part-1/section-1.401(k)-3

• Internal Revenue Service. (2026, April 8). Retirement topics: 401(k) and profit-sharing plan contribution limits. https://www.irs.gov/retirement-plans/plan-participant-employee/retirement-topics-401k-and-profit-sharing-plan-contribution-limits

All retrieved 2026-11-11. Dates in parentheses are each page’s “last reviewed or updated” stamp.