1. There is no specific licenses needed for conversion. FAS62xx comes with own licenses and you are supposed to install them after upgrade. If old system has more (or different set of) licenses, you are supposed to delete those, that are not available for new one.
2. Head swap is always disruptive with 7-Mode.
3. Tough. See below.
As of now there is no real documentation I am aware of dealing with this. Simply booting 8.x on root volume with 7.x may work, but I have not seen official statement of support (if someone can correct me - excellent). The most bullet proof way I can think of is to loan a head supporting both 7G and 8.x and do two consecutive head swaps with Data ONTAP upgrade in between.
The task is simple to perform but non-trivial to plan. If you do not have good experience with NetApp it really makes sense to arrange for professional service to do it.