Afaik, only developers can change milestones/lables on existing issues.
A reporter can apply an initial value when creating the issue (aka suggesting values).
We are still sorting out which Labels to keep.
As far as version goes, only the "Version: ..." indicating in what version the reporter found the issue are currently deemed to stay.
For target, the milestone will serve.
As for any sort of priority, those existed in Mantis but were not used there. So unsure if they are going to be kept, and even if, they will probably have little impact.