XYZ Company adopted a prototype document provided by financial institution A. Financial institution A is in the process of being taken over (purchased) by financial institution B. XYZ Company was told that they would not have to adopt financial institution B's plan document because financial institution B will simply become responsible for the document that had orginally been provided by financial institution A. Was XYZ Company given correct information?

Sounds vanilla to me.

Since it appears that the takeover hasn't happened yet I would ask for something in writing from B indicating that they will continue the plan document that A used. Sometimes things change from how they start in situations like this.

With that said, why not look at the document for A and B and see which one you would like to use when updating for GUST.