From the r/kotlin Reddit, user Nek_12 shared their experience with migrating to Android Gradle Plugin 9.0 in their 150,000 line project and it seems like it was a bit of a nightmare:
Yesterday I finally finished migrating a big 150,000-line project from AGP 8 to AGP 9. This was painful. This is probably the biggest migration effort that I had to undergo this year. So, to save you from the pain and dozens of wasted hours that I had to spend, I decided to write a full migration guide for you.
Be prepared, this migration will take some time, so you better start early. With AGP 9.0 already being in release, Google somehow expects you to already start using it yesterday. And they explicitly state that many of the existing APIs and workarounds that you can employ right now to delay the migration will stop working in summer 2026. So for big apps, you don’t have much time left.
Before we start, please keep in mind that despite AGP somehow being in production release, a lot of official plugins, such as the Hilt plugin and KSP, do not support AGP 9.0. If you use Hilt or KSP in your project, you will not be able to migrate without severe workarounds for now. If you’re reading this later than January 2025, just make sure to double-check if Hilt and KSP already have shipped AGP 9.0 support.
Reddit: https://www.reddit.com/r/Kotlin/comments/1qi1117/agp_90_is_out_and_its_a_disaster_heres_full/
If you are working on a mature Android/Kotlin project, you may want to hold off on upgrading to AGP 9.0 and assess what changes you will need to perform prior to migrating to AGP 9.0. There is a live tracker here to see which dependencies have been updated to be compatible with AGP 9.0:
Link: AGP Plugin Status