Best way to revert to Sierra from High Sierra??

[prof]

Thanks for the reply. I'm guessing that the upgrade will be OK but trying to be on the safe side. I did find that there's a way to bypass the APFS conversion when installing High Sierra. Is it possible to do an upgrade when using the regular High Sierra installer??
The client doesn't want to do any software reinstalls which would be required if the High Sierra installer does a clean install of the OS.