When is merger effective and how to report?

Company A buys Company B and merges the Plans effective 1/1/2025.  Assets of B are liquidated on 12/31/2024 and received by Plan A on 1/2/2025. 

Would you complete Plan B 5500 with transfer out and ending balance of 0.00 on a final 12/31/2024 5500? Or would you file a 5500 for January 1 - 2 and show transfer out to 0.00 on Final 2025 5500?  If the 2024 Forms for A and B were already filed without recognizing the transfer, would you amend the return(s) and if so, in what manner?  

If Transfer Out is shown on 2024 Form for Plan B, when would you show on Plan A 5500? On the 2024 Form or the 2025 Form? 

Thanks

There has been prior discussion on this topic.  Perhaps the Search feature will help you.  There was also a Q&A in a prior Gray Book, and a verbal Q&A at a prior Enrolled Actuaries meeting.  The IRS is sympathetic to such timing issues, so you can use 12/31/24 as the final date for Plan B for all purposes.

Note that one second after midnight on 12/31/24, the Plan B assets are owned by Plan A, no matter where those assets are located; the "received" date is not relevant.

Thanks, I tried the search first but my results are always so broad that they are unhelpful.  Will try again. 

 I learned a bit more on search and found two threads. Posting links here in case anyone else interested:

Merged plan Form 5500 requirement - Form 5500 - BenefitsLink Message Boards

Plan merger and final 5500 - 1/1 merger date - Form 5500 - BenefitsLink Message Boards

Here is another thread: