Upgrading from very old Site Builder
Using Jamroom
Its real hard to say what will happen without doing it, then fixing the results. Probably after the import you will need to fiddle with things.
The worst thing to do is to change it, see things are not quite right and immediately revert because then you'll have an extra step (which involves database manipulation) next time you want to try, so set aside a few hours just in case. do it, and work through any issues after the transition.
If it all does fail and its not going to work, you can turn off the new sitebuilder module and turn back on the old ones and you will be back to where you are now.
You probably will find stuff that doesn't line up exactly, fork it and add your fixes where you find them.