axel swap/brakes qustions

according to my mechainc the rear end is goning to fail at some point so i was thiniking of swapping the whole axel once it breaks to a 98+ g80 unit with 3.42 gears (mine is a open diff with 3.42 1995 M.Y.) that way i can get the added benfits of disc in the rear i know they have covered the alxe swap on thse fourms my qustion is do you need to change the piston on the rear portion of the break booster,,due to the fact i heard disc reqires much more pressure then drums so does that mean i should take the booster master cyl. whatever it is under the hood from the one with the disc and swap that as well?