Same with caulking the roof. They recommend you clean existing caulk with a mild solution and then let dry completely. If existing caulk comes off easily then remove and re-caulk. If it does not come off easily, then caulk over it with their recommended caulk.
Rick and Mindy
2013 Redwood 31SL, 2011 Ford F350 SRW, 6.7, Crew Cab with Firestone Bags, Titan Disc Brakes