Depending on your sources, those classes have been called all kinds of things (explorers/cruisers/battleships/etc).
To my mind, I don't see the Sovereign as a follow-on successor to the Galaxy. Two different types of ships with different mission requirements and specifications and different roles.
The Sovereign class isn't a "new and improved" Galaxy class. It's a different kind of vessel with a different role, better in some ways, not in others. Sovereign doesn't necessarily make the Galaxy obsolete. They might even serve well together in a complimentary manner.