Merging welfare plans

[TPApril]

Plan sponsor wants to end one welfare plan on 12/31 and merge it with another existing plan on 1/1 (the next day). For 5500 purposes this would mean a terminated plan is ending the plan year with participants. What is the general best practice way to do this? My preference would be to terminate and merge them on 1/1.

Bonus question - can a welfare plan be terminated on 12/31 with the resolution being signed the following calendar year?

[Griswold]

If you terminate something, you can't merge it. You can terminate one and put all the participants in the other plan, or you can just merge the plans--probably on 12/31 to avoid costs.