Is this a calendar year plan?
Option 1 - that you seem to be proposing?
I'm not sure I think there is a missed opportunity to defer - but let's assume there is -
Rev Proc 2019-19 provides that no QNEC for a MOD is required for the deferral portion if the missed period is less than 3 months. So as long as folks are notified of the update and have the opportunity to change their deferral election for any paydates on or after 4/1, I think that is fine. There may be some SH match due if they participants don't max out the match on their own, but I don't think that's the case either. Mainly because I don't see this as a missed opportunity to defer.
Option 2 - which I think is more likely, and how I would approach it
The IRS has provided explanations about mid-year changes to safe harbor provisions. And they are generally allowed for match if there are at least 3 months left in the year (check), is retroactive to the beginning of the year (check), and increases the benefit (I would say an increase in who is eligible / decrease in eligibility conditions is an increase in benefit), and have an opportunity to change their deferral election. There is no mention in the IRS guidance about making sure participants are made whole because of some missed opportunity to defer. Likely because if there are several months left in the year they (in theory at least) have the ability to adjust their deferrals and receive the increased match.
I'm sure others can delve into this more deeply or maybe chime in if there is some nuanced rule that applies to change the analysis, but Option 2 is how I would analyze it and treat it.