Moving from legacy fields to database extensions is complicated. There are fields we aren't allowed to move, fields that were already moved, and fields that will be created by core migration 1 and 2. Looking at a list of legacy custom fields doesn't really tell you which ones are available for migration. CLG's Enhanced Legacy Fields is a customization that aims to help.
What does it do?
It updates the 4 custom field pages (students, faculty, courses, sections) so that the fields are rearranged by status.