PracticalERISA

Since you can actually adopt a plan with a calendar year on December 31, retroactive to January 1, you do not have a retroactive amendment problem if your participating employer (X Corp) signs a Participation Agreement before the end of the year. The Plan Sponsor and X Corp should each sign resolutions - the sponsor authorizing the participation by X Corp and X Corp formally adopting the Plan. I am assuming you meant to ask if the merged employer needs to sign a new Participation Agreement. If both Y and Z were participating employers and Y merged into Z, then you have no need to have Z sign a new agreement - it simply has more employees who participate. If instead, Y and Z are merged into NewCo, then NewCo must sign a Participation Agreement. In any case,I would also memorialize what happened by adopting a resolution of Sponsor formally ending Y's participation in my first scenario and both in the second. You should ask these companies for documents because they often call an asset sale a merger.