2
That is a very good question.
Whatever you do, remember to make some backups (of both files and database) first.
I really don't know which changes to the database have happened since XOOPS 1.0 RC2, but if you manage to get to XOOPS 1.3.10, you can use the installer of XOOPS 2.0.x to upgrade.
Not a simple task at all
Good luck.
"When you can flatten entire cities at a whim, a tendency towards quiet reflection and seeing-things-from-the-other-fellow's-point-of-view is seldom necessary."
Cusix Software