I did not realize it is 'not' 32bit compatible 'at all'. Zip, nada, nothing.. I lost 3 rather useful programs I found have not been re-written (and probably won't be) to 64bit. One is my program to run Windows Office on my Mac..
After a few attempts myself to restore back to High Sierra, I did end up going to the Genius Bar and having them do it. What a PITA!
Fair Warning! Be certain you have no 32bit programs you hope to keep before you upgrade.