Stu, I think he means replace...
How easy it would be depends on whether the boat ever had a marine toilet and holding tank, and therefore already has the necessary thru-hull for flush water intake, deck pumpout fitting for the tank and vent thru-hull for the tank. Worst case, you have to have the boat hauled to install thru-hulls/seacocks, find a location within 6' of the toilet for the tank...cut holes through bulkheads for the head discharge hose to the tank to pass through... Or if the boat previously had a marine toilet and tank, just service the seacocks, install the toilet and run the hoses. And then there's the matter of what type of toilet--manual, electric...if electric, one designed to use sea water or onboard pressurized fresh water...and your budget. If your portapotty is a 5-6 gal model that's permanently installed and fitted for pumpout, you may be better off keeping what you have...'cuz you'd have to spend at least $500 for just the materials to replace it, and you'd need at least an 18-20 gallon tank to gain any advantage in the number of flushes it holds.