Calendar Year MEP Splits Mar-31 & the 5500 Considerations

Two related companies filed their MEP 401(k) plan's single 5500-SF annually over many years. The common ownership ended, and the MEP split into two separate plans effective March 31, 2024. I am trying to understand how to handle the 2024 5500-SF reporting for one of the companies - the one that became my client just recently. 

Was a short plan year filing due for the MEP? I do not think one was done. Can either company file a full year 5500 for 2024? What should be considered in order to make these determinations?

Typically one would remain the sponsor of the current plan and the other would spin off into its own plan. The spin off would likely be a short plan year unless crafted carefully at 1/1.  
 

But that’s just the way I would have done it. 

I agree with Bill, you would normally have a new plan for one of the companies since the other would remain the sponsor of the original plan.

If you did two new plans, the old MEP would need to terminate and file a final 5500.

I discovered that my new client is the one that is continuing on as sponsor of the original plan. However, I also discovered that the last several 5500 filings show an effective date in 1994 while their plan document shows an effective date in 1986. Month and day are also different. I have confirmed where the error came from, though I have not previewed all the filings to see how far back it goes.

Correcting all the previous returns (could be 30 of them) would be a mess. However, that is the correct way to handle this. Right?

My 2 cents on the effective date - I would just start using the correct effective date going forward and have documentation ready for any questions later.