I don't think it's posible to ISIN from 5.5 to 6.1
Probably not want you want to hear but I would not short cut the upgrade, follow the route Northgate advise and support.
Although I think for you it's just an easy ISIN the newer version VIP and regen the system.
However, we have been thinking about attempting to install some of our smaller projects straight from our VIP 5.5932 into 6.1 to reduce some overhead (do not ask me why we do not just upgrade our VIP environment to 5.5932 or higher, I would, but I don't make that call).
Still it is tempting to go with the short cut and if you have issues where Northgate suggest going back to the original codebase and redo the upgrade, you can do a nice "I told you so" to whom ever does make the call .
Of course it might all be fine.