I have never heard of a migration from InstantForum to YAF, so you'll be the first! Make sure you post back as to how it goes, problems, issues, etc..
To start you off, have a look at the guys migrating from PHP forums, while the code will be different, the process should be the same. Your in for some serious "manual" DB work, but keep at it. SQL Server 2005 has some great FREE "Integration Service" tools that will help you match your old tables (and fields0 to new YAF tables.
.....the man in black fled across the desert..........and the gunslinger followed.....