Moving Exchange Account Source Account

I have a very complex environment I'm hoping someone might jump start my search. We have two domains syncing to Entra ID. One domain is a resource forest where our Exchange environment sits. That domain contains disabled stub accounts synced to our primary domain where the actual user accounts sit. The source for all EXO mailboxes are the stubs in the resource forest. Those accounts are kept in sync using FIM 2008. We're wanting to decom that entire resource environment and move all of the attributes to the primary domain. The resource domain schema is the last version of Ex 2016. The primary domain schema is Ex 2010 SP1. I know my first step is to update the primary schema, however, has anyone encountered a situation like this? Any help would be greatly appreciated.