First off, wait three more days so that the Ultimate Collection you get will include the upcoming new expansion pack.

With that said, I'll assume two things:
  1. You're playing on PC
  2. The install process for Ultimate Collection Seekers Edition will look like the install process for Ultimate Collection Abyssea Edition

If both hold true, then if you buy the Ultimate Collection you will be able to pick and choose which expansion(s) to install from it. You'll still end up downloading a few expansion-exclusive updates when next you log in, but it won't be nearly as bad as a fresh install of everything.

EDIT: I've no experience with the Steam version, so I could be wrong.