Yes this will be disruptive, but it was long overdue.
The "pinnacle" of raiding was an utter mess due to the 25 vs 10 balance issues and constant arguments over difficulty/gear.
That is what it should have been in the first place, a single size for the top-level raiding of a given tier.
Blizzard are now doing what they should have done a long time ago.
I understand why there was the 10 vs 25m split, to be as inclusive as possible but in the end a variety of issues simply messed it up.
Partly the community, partly the fundamental flaw in trying to balance the two.