...Because they should.

The large overhaul of the combat system should be implemented a few weeks prior to the expansion launch, to give players ample time to adjust not only their gameplay, but their hotbars and HUDs as well.

This method of implementing combat changes has been wildly successful and well received in other MMORPGs. Not only does it help the player base grow comfortable with the changes, but it also adds a reason for players to return and "prepare" prior to the big day, whether that be the running of then-old content with the new changes, or experimenting with the reworked jobs to get a feel for what you may want to "main" once the expansion hits.

I hope that in this specific case, SE takes a page out of their competitor's book of success.