Yeah, I also would recommend a clean new install.
As
drsnx60 said, backup your stuff first, perhaps do a full backup, and then do a new installation, maybe on a fresh drive.
A new installation will take you less than an hour (not counting backing up your data, which you have to do anyway) while those many upgrade steps will take you days. And a zillion things can go wrong while this stunt, since several things changed since 12.4, like default shell changes from csh to sh - overseeing such thing, and you also have a mess to clean up, too.