If it's a vanilla install of .classic there shouldn't be a problem converting. It might take a day or 2 but it's just a matter of reading -> writing, reading -> writing, a million times or more.
If the server is dedicated, probably will need to increase the amount of memory available to php/mysql during the import + Slow down the process so the reading doesn't get ahead of the writing (the hand-holding SD was talking about). In most of my imports, if it hiccuped you just had to back up one screen and it would go again.
