Unfortunately I'm a MSSQL/C#/ASP.net and Flash/Flex guy, so my MySQL experience is pretty limited. In terms of the actual conversion process, it doesn't look too bad (especially if you're doing it on a backup) based on the process you linked to. Since SMF does have a process in place for the conversion, it doesn't look you're exactly in the wild west - I'm sure others have paved the way to some extent.
Of course, if you don't have a backup and mess things up, this wonderful forum is SOL - this forum is backed up, isn't it? My guess is that only a few existing posts might get some ?'s or a few ?? -type things where they used to have a non-english character if the process isn't perfect, but then again, it wasn't really possible to put an umlaut (?) in a post as it stands now (at least in Safari on a Mac - maybe it would work if I used my a Windows machine, but truthfully I don't know how to make an umlaut in Windows offhand). Pretty minor collateral damage, and I seriously doubt any performance difference would be noticeable.