Can you describe the error you see when testing (if any) in more detail please?
Considering you have both old and new environments in place I'm pretty sure this is a routing issue where the servers are configured to route traffic back to clients via the old F5 pair. Check the servers and confirm if there are static routes configured for the SNAT range (of course it could rely on a default)? Use a new/unique SNAT range on the new F5 pair and configure statics routes on the servers for that range pointing to the new pair. You get the idea...