For modifying an environment variable, please clone the entire path MSI and use an Environment element to update it. Should be a little simpler than that one, because it's a constant.

I'm always very hesitant to modify system-wide (or user-wide) settings like this though. I'd be more comfortable if we made it a PYTHONUTF8_310 variable that _only_ applies to that specific version. Otherwise someone might install an update and break existing apps.

Alternatively, for this kind of policy option, it might be nice to set up some common registry keys to define it. That is a function I've been thinking about for enabling some security features (such as malware scanning), and it mimics how Windows distributes its own configuration already. That'll take a whole PEP, though, and I doubt I'll get it ready in time for 3.10 given the other stuff I've got going on right now.

So provided we adequately document that the setting may impact other versions of Python besides the one being installed, and it's not enabled by default, I'm okay with an MSI-based addable/removable setting for PYTHONUTF8.