A person that desided to switch between squid and squid-novm and removes one of them before installing another will be very happy to discover that his/her whole cache is gone. I know that a heavily loaded squid will probably regenerate cache in several hours, but it may take several weeks or even months for a lightly loaded cache to get back to its original HIT ratio after a cache loss.
(assuming you mean *not* very happy :) ) Fixed in squid-2.1.PATCH2-3. Also, logs are no longer deleted at uninstall.