If Company A is merging with Company B and Company A has excess plan assets in a funded welfare plan, can those plan assets be used for the benefit of all new employees (those of both Company A and Company B). Unable to find guidance on point...any ideas?

This is a bit of a sticky wicket because the answer depends on several unstated facts: (i) what do the plan docs say about mergers and acquisition?s; (ii) what do the plan docs say about plan termination and excess benefits?; (iii) does (or can) Company B have a welfare benefit plan to be merged with Company A's plan?
These issues should be addressed and a strategy developed prior to any merger/acquisition, or everyone's hands may be tied.