The first step is to trigger a SuperDuper bootable backup of the system, just in case. I have this super-cheap silicone case external drive case I use with bare drives to do all this stuff. Once that backup is complete, it’s Mojave time.
Still, High Sierra does have the killer feature I wanted - AN EXCUSE to reinstall macOS. I’m just going to move forward with that. I use the process to create an bootable installer and I use my 250GB external micro drive. Ever year I think, “I really should just have a 32GB thumb drive” but every year I don’t have one and I just work with that I do have. You’d think at least this process would be easy, but that external installer just keeps stalling out on me and It’s been a couple hours tonight, so it’s time for bed.