Well, anyone who isn't regularly backing up their own site, and thus already knowing how to do it, is playing with fire. Keep in mind that LP doesn't keep its server-wide backups for more than a few days, and then charges $75 a pop to restore anything (last time I looked). If your site gets hacked or corrupted and you don't notice for a week or so while you're on holiday/vacation, you're out of luck if you're depending on LP's backups. I don't ever recall hearing of it happening at LP, but at other hosts, they've tried to restore crashed servers only to find their backups were no good!
Personally, I think a reminder to back up your site before the move is adequate. If someone isn't interested in learning how to do it, they're certainly not going to be bothering to learn it now. There's certainly no need to include instructions with the email, although a Wiki page link on best backup practices would be useful (see the Simple Machines Forum wiki page Backup, which I co-wrote).