I don’t recall having seen anything exhaustive, but maybe someone’s got a conversion script or gist. Instructions can vary by platform, so if you find something like that, I’d make sure to verify it makes sense to run on your OS before giving it a go.
The “safe” route is to just uninstall, and reinstall. (The work described in Anyone up for picking at some Nix ~onboarding improvements? would make it fairly simple to achieve this.)
Since you’re asking about how to change it, I’m guessing you are hoping to avoid an uninstall, probably to preserve the existing store? (The installer might be able to directly support this via a flag after the work linked above, but I am shooting from the hip…).
If that’s a must and you can’t find a current conversion script out there, you’d probably need to piece together the differences from the single/multi-user install scripts (in the nix source’s scripts directory) to figure out what to remove and what to add.