Likelihood is that in operational terms the MARS ships will replace the Fort classes although they will remain in commission. Presently one Rover is allocated to FOST with the other and the Orangeleaf doing non-direct naval support work such as refuelling Gibraltar and other offshore locations, although both replenished the fleet in Libya which as far as I am aware is the closest they have got in recent years and that was due to their proximity. Otherwise they are refuelling and supporting the Caribbean ship and the South Atlantic ship whilst the Forts are usually in the Gulf or with the carriers (Response Task Group or whatever it is called now).
My guess is that one of the new MARS ships will take over FOST and will also serve as the class trainer for the RFA whilst the other three will be divided around the Response Group and Gulf alongside the Waves...
I don't know where you are getting your info from, but it's way off.
The MARS FT ships cannot replace the Forts operationally, and are not intended to. The two Fort AFSH are purely dry stores ships, MARS FT are tankers, it just doesn't go. Fort Victoria has been operating as a counter-piracy platform since she went east, MARS FT doesn't have the aviation, boat or personnel capacity to take over that operational role either.
Orangeleaf is FOST tanker at present, the Rovers take turn-about on APT(S) and relieving Orangeleaf (Gold is down south at the moment, Black is in refit) This is likely to continue until MARS FT enters service. We don't send single-hulled tankers to the Caribbean and haven't for many years and apart from Fort Vic we won't send any to the Persian Gulf or vicinity either (last one was Bayleaf which returned to in early 2011). I haven't heard of an RFA doing a freighting run since Oakleaf decommissioned, we charter Maersk for that. All RFA tankers are primarily replenishment units.
Waves will be rotated between APT(N) and AGRT until MARS FT arrives.
One Fort is usually assigned to RFTG, one bobbing about elsewhere and recently one has been at extended readiness. Apart from Fort Vic's counter-piracy deployment they tend not to be East of Suez as singletons.
As MARS FT arrive I expect them to start by relieving the pressure on the Waves with FOST and APT(S) being the last tasks they take over from the SHTs.
SDSR 2015 is an undiscovered country.