Converting (it's more of a conversion than a migration) from flexible GAE to cloud run is super easy, check out the conversion tool I posted in my previous comment.
Basically, your code shouldn't really need to change at all, it's really just your deployment scripts and configs that need to be updated. At their heart flexible GAE and cloud run are both just running Docker containers.
GAE Flex is super old at this point and I've never personally met someone who migrated between them (they're pretty different offerings imho). Moving between either GAE to Run has been pretty seamless though.
