Well, few minutes ago I tested it on a big forum 4.1.10 (20K users, 25+K threads, close to 100K posts) and everything is still working
I am not so sure if it was actually converted or not, the reason I say that is because I have few java scripts that I expected to have issues with but I didn't !...
I will check everything later today and report as I need some sleep now.
EDIT: Checking db via PhpMyAdmin I see it did convert to UTF8... Thanks