I'm w/ jpod here. It appears ("stock purchase") that Company A bought Company B "lock stock and barrell". If so, then the buy/sell agreement need not mention any plans, because B remains the sponsor of its plan(s), but now B is a subsidiary (or something similar) of A.
If this is an accurate summary, A has no ability to "not accept" Plan B. A is (probably) now a fiduciary of the Plan B.
Stated another way: Plan B has not changed, but Company B now has a parent company.
Yes, they have the option of merging plans, but careful review by competent advisor(s) is warranted. (Just guessing: did any lawyer look at this in advance?)