I have a 07 OBV with gravity I (center locker bag). Have been filling rear sacs manually for surfing but looking to upgrade to auto fill/reversible system i.e. gravity III. Any advice on best way to upgrade. Can I utilize anything with the gravity I or do I simply need new system for the rear. Mostly surf and really looking to maximize one side fill in rear as all our riders go regular. Have had decent wake but I know we really need more weight. Thx.