If I buy an old boat which does not meet present day head requirements, is there a grandfather clause which prevents me from being required to up grade?
The no-discharge regs don't work that way Nohiking. The rules are quite simple in their intent and cannot be sidestepped by convoluted semantics or pre-existing head setups.
Essentially, in both USCG and our state RIDEM regs (Rhode Island Dept of Environmental Management), if the boat has no means of directly discharging sewage overboard, the environmental enforcers apply a green sticker to the exterior of your boat. This signifies that marine toilets can only empty into an onboard holding tank which requires pumping out by a regulated pump-out facility.
If the inspected boat has a direct discharge through-hull, capable of expelling sewage into coastal or inland waterways - up to 3 miles beyond US coastlines, the vessel is issued a yellow sticker. This identifies the boat as one which must have the means of locking the through-hull valve handle for each head and/or holding tank, while within regulated waters.
It doesn't matter when the boat was built and what type of heads are onboard. We simply cannot "directly" discharge sewage from
toilets or holding tanks.
However, the irony is, it is unclear whether a person can legally defecate or urinate into the water by bypassing the toilet all together. If this was enforced, we'd need to lock up all sea life within the 3 mile limit.