In my (limited) experience: not by a long shot. Heroku setups may seem locked in, but GAE is obscene. No outgoing HTTP requests, no normal DB, basically everything is GAE specific. Heroku is not the most generic thing out there, but, by God, it's still miles better than GAE.
The tradeoff is super interesting: using their framework allows them to hella optimize the instance for you, but you're pretty darn stuck with it :/
Also, GAE can now run docker and also connect to MySQL running on Google Cloud infrastructure, which is a separate offering.
So in GAE too, you just do it.
Most of that would just be setting up the new one and migrating the data across.
You don't have to significantly alter apps (in that stack) to be heroku-compatible.