Can I change payment timing under a "Short-Term Deferral" plan to a different date (where the payment remains a "Short-Term Deferral"?

We have a Plan that provides benefits vest on 1/1/2022 (if the participant is employed) and are paid within 30 days thereafter (so, within the short-term deferral period).  Clearly, the Plan doesn't provide for nonqualified deferred compensation, and 409A doesn't apply.

We want to amend the Plan to provide those same benefits vest on 1/1/2023 (if the participant is employed) and are paid within 30 days thereafter (so, still within the short-term deferral period).

Can we do this?  It seems that we are just switching from one 409A-exempt arrangement to another, so I can see the argument this doesn't ever implicate 409A.  But I also see the opportunity for abuse here, and I believe I've seen commentary on this before (I just can't find it now).

Bonus Question: Would it be any different if we were accelerating the vesting/payment rather than deferring it further (but still keeping it within the short-term deferral window)?

Good question. My quick take from memory:

As long as this is the employer's choice, you are OK.

However, if the participant made the choice (or had a lot of input, which is facts and circumstances), you should comply with the 1 year/5 year rule to postpone a short term deferral from one calendar year to another.

In the case of accelerations of short term deferrals, that happens all the time and should be OK. 

I see a few alternatives:

• If the employer can truly do this unilaterally, even if the employee disagrees, there may not be a legally binding right to the payment and 409A is not implicated. My guess is this is not the case (i.e., there is probably a written agreement with the employee that would require the employee's consent to move the payment date). 
• Assuming the above is not the case, the employee's consent would be required to delay vesting. In that case, I think you are stuck with the "addition or extension" of a substantial risk of forfeiture rules, which would require the later (2023) payment to be "materially greater" than existing (2022) payment. Usually 25% is a good rule of thumb.
• If the amount will not be materially greater, I think as mentioned above you will need to use the 1/5 year rules to make a deferral election of a short-term deferral.
• Accelerating the vesting (and therefore 30-day payment window) date is usually fine.