Is there a way to upgrade from Ubuntu 11.04 to Crunchbang without a full installation? I'd like to keep the files on the hard-drive intact.
No, there isn't.
At best, if you're reasonably neat, you'll be able to keep
/home intact, but despite the fact that they're both Debian derivatives they're entirely different distributions.
Your "upgrade" is the rough equivalent of asking whether it's possible to turn a fedora core install into a SuSE one.