Thank you for your replies. We got back to the accountant for more relevant information:
A was dissolved on 6/30/2016. B was formed on 7/1/2016 and assumed sponsorship of A's plan effective 7/1/2016.
There was no asset sale or stock sale.
All employees were terminated by A and then hired by B, with the exception of the former owner of A who started new company C.
So, to re-word our original question, is it possible to amend A's plan to fully vest only former employees of A who were not hired by B?