It's actually not bad at all - I can't speak for the java side of things, but transitioning a couple random nodejs services to lambda took under ten minutes eac

It's actually not bad at all - I can't speak for the java side of things, but transitioning a couple random nodejs services to lambda took under ten minutes each, with most of that time being testing (my only complaint is that the logging sucks). Lambda function are so modular that I can't imagine not being able to trivially slot them back in to a standard express endpoint. It might be tricky if you do a bunch of stuff with S3, but even then it shouldn't be terribly difficult to sub in the standard AWS modules.