Blizzard is most definitely NOT going to revamp all previous mounts with animations for Dragonriding, like get fucking real!
Also, it's most definitely NOT going to enforce Dragonriding in all previous expansions areas because that will make all older flying mounts unusable.
There are only 2 options here:
1. Dragonriding is just a Dragonflight expansion feature, same as Torghast, and it will stay in the Dragonflight areas.
2. Dragonriding becomes an option alongside regular flying, that offers faster flying but requires a more active control of the mount.
If you ask me, I think option 1 is the most likely to happen.