Is there a utility that could help us to migrate the users,config and data from a 8.x installation to a plan 9.x installation?
there is no such migration tool available to do such a migration. Manual instructions to do such a migration are available here:
Please note: You can't skip any major releases during such a migration. If you're currently on e.g. 8.0.14 (8.x in your topic title is too unspecific, there are three major versions in 8.x) you can only migrate to 8.1.9 before doing the needed upgrade steps to 8.2.x, 9.0.x and 9.1.x