If XP were written in Lisp and the person had a terminal connection of some sort to the OS REPL, then reasonably, it could be done. If the OS were well-written and designed, a major change could be enacted within a minute, although anything more than a few changes might require some automated development tools and maybe even AI to help -- not so unreasonable in any Gundam universe. Then again, Lisp lends itself to creating powerful abstractions that allow you to manage lots of functionality with minimal code, so a genius coder might be able to get by without any extra tools.
If the whole "changing things on the fly" idea really seems that unreal, consider a real world example. Viaweb was an online web application for building stores written in Common Lisp. Before Viaweb was bought by Yahoo! (making Paul Graham and the other founders rich), they used to debug problems and fix them on the running web server within a matter of a few minutes. They'd fix problems while users were still on the phone with them for support, and almost make them believe there wasn't a problem to begin with. No interruptions.
Or better yet, just look up as much as you can on Lisp Machines and developing on them.
It's really not as far-fetched as it seems.