It's only a lot of change if you've not been using the late 3e books, particularly Tome of Battle. If you are, it's practically no change at all.
I don't think it will be that hard to convert. A 5th level fighter with a 16 str and a 15 cha who uses a sword and shield becomes a 5th level fighter with a 16 str and a 15 cha who uses a sword and shield. An encounter with one troll becomes an encounter with five trolls.
There are still going to be monsters living in holes in the ground being beseiged by small bands of incredibly heavily armed killers. Practically all of the 3e system remains. We even still have minutiae like swift and immediate actions.
Look at the change from core OD&D in 1974 to the 1e PHB 4 years later. Elves, dwarves and hobbits became races instead of classes. Seven new classes appeared - druid, paladin, ranger, illusionist, thief, assassin and monk. And the first prestige class, the bard. Several new races - gnomes, half-elves and half-orcs. These are far more extreme than the changes between 3e and 4e, which took place over 8 years, twice the time gap.
Now you might say that these changes were far less extreme if you were using the OD&D supplements such as Greyhawk and Blackmoor. But that's exactly my point. The same sort of thing is occurring now as in 1978, but to a lesser degree.
If 4e isn't D&D, then 1e wasn't D&D times four, because it was twice the amount of change in half the time.