I think the helicopter aft more or less replaces the limbo + magazine aft, but not the missiles. replacing the boilers with gas turbines might save weight, but would cut range and the machinery being in the middle of the ship saves very little actual weight forward and aft but will give you space to put some ballast aft to get her back on displacement. I just noticed your sea dart is 2 full decks higher then it is on the real world T82, the weight of that almost certainly would get you stability problems
may I suggest lowering it, as you had done on one version? note also that your current magazine is also smaller then that of an T42 (you have drawn in the same size but it goes into the Ikara pit as it is now, so it will be smaller in reality), was that intentional?
shippy, not trying to burn you down but you have chosen the hard way out
changing an 'working' ship into something else takes a lot more effort and time then you would think, sometimes (and I think this is one such) more then designing from scratch.