Personally I would rather pay for each major update of VB4 than to move to VB5....did few posts recently on vb.com and...:
my noob forums are more functional
Hope one will help you with that migration, but if its like less than 100 users or 10-20 active ones just let them know and do new fresh install...in future it will be easy for you to upgrade if needed
Let us know how you did it, share info with us :]