Paralysis by analysis?
First off Id add or step up to the new electronics and see how they work with whats on hand.
One move at a time is also a more insightful way to predicate decisions. Its easier and simpler too.
After that consider the needs of the newly proposed speakers if any are then still desired/needed.
Adding speakers to amps is a key thing to do and certain parameters need to be addressed and satisfied. Following that your preffs and budget will determine which are the ones you can get.
Staying in the same makers ball park for HT is a very good idea. Of primary importance is for the front 3 to be as cohesive as possible. Surrounds could be moved in & out from other lines if necessary. The info there is getting better but its still ambience more often than not.