Will my OS extensions break on AC migration?

Most don’t. The breakage rate I’ve seen:

• Hyvä + Hyvä-compatible extensions: 100% carry over, AC and OS run the same Hyvä.
• Plain OS extensions from major vendors (Aheadworks, Amasty, Mageplaza, Magezon): ~95% work unchanged. Edge cases: extensions that monkey-patch core checkout or admin grids may collide with AC’s versions.
• OS B2B extensions: ~50% of these duplicate AC’s native B2B modules. Plan to remove them and migrate data into Magento_Company tables. Don’t leave both running.
• Custom modules: nearly always carry over. The collisions are with AC-native features (Staging / GiftCard / Rma / CustomerSegment), if your custom module duplicates one, refactor.

Practical migration plan: pre-flight audit of every installed module against AC’s module list, decide remove / keep / replace per extension, run side-by-side regression in staging. 1-2 weeks of focused work for a typical 30-extension store.