I don't know 1.3, but here are some poignant tips:
1. Backup you current files, then backup your database.
2. Always do #1 first.
3. Restore your 1.2.x backup files to your online directory... you did backup the files didn't you?
4. Then find out what changes the upgrade made to your database and how to reverse or undue those changes.
Per #4, they probably have a tool here for doing that and keeping the forums, topics, and posts that have occurred in the interim.
Per #3, it is always best to test an installation either locally (on your own server), or in a separate installation online while you sort out the particulars and run your tests. Then take it live, after backing up the live site files and database first, of course ;^)
Well, this doesn't help you with the database chagnes, but I hope it helps in general.
Good luck!
TwoHawks
Love is the Function
No Form is the Tool