To the best of my knowledge, there wasn’t much of a difference in programming style going from GC to ARC except for a few exceptions like all those pesky CFMakeCollectable calls that ended up being no-ops anyway.
You don't have to explicitly mark things as strong as it's the default. Having to manually break references cycles with weak pointers does make the migration not as simple as just changing the compiler flags, but I've never heard of it being all that difficult. Apple managed to migrate Xcode in a single version.