It is possible to re-define top-level symbols with the REPL currently. I haven't had the opportunity to try live coding a game but replacing top-level definitions is something I did a lot when developing geiser-hoot.
Viewing post in Make a browser game in Scheme with Hoot
Thank you. I tried the REPL again, and after importing a module, functions can be redefined. What I was trying to do was fetching the function name and redefine it (using set!) in what I had assumed was the main process, I understand know that run-web-repl starts the REPL from scratch (with very little context from where it is launched). I will try to start the game loop from within the REPL and see how it goes.