Earlier quoted context omitted.
If you're just now looking in to GAE, you should likely be using Cloud Run instead. My company is busily migrating everything there and reaping the benefits.
So, this what scares me: in 5 years someone using GAE would be busy with two migrations: classic GAE -> flexible GAE -> and now cloud run?..
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.