I just had an issue in which I changed the repo of a package I had been using (use-package), to a fork of that package. Making that change in Straight proved that either there needs to be an easy way, or I need to know the easy way. Due to conflicting recipes I ended up changing the use-package statement (expected), editing the straight cache file (ugh), manually deleting folders in /repos and /builds, and restarting emacs.