You won't be able to port it without any additional work, but if you are willing to do some refactoring (specifically removing the Lambda-specific objects, etc) then it can run anywhere. Of course the time this will take depends on the size of the project, but it can be definitely done without rewriting the whole application, but just rewriting the "invoker" part.
To make a future port easier, the functions could also be modularized by keeping the real business logic and the Lambda integration separate, but integrated.
This is a huge risk. If they shut you out, for whatever reason, you're done. You can't port your code to somewhere else. It's Lambda specific.