Disney could do whatever they want, even with land constraints. Look at THV, something no one thought they could ever pull off, much less make part of SSR.
I bet somewhere in a file cabinet are plans for DVC at both Poly and GF...including how to do it. Now, if we ever see these plans could be debated.
In the case of Poly, I imagine they would start with the outer most buildings and possibly complete rebuild or gut and converting to DVC. DVC always seem to get the rooms furthest away....and there are numerous reasons for that (least desirable to cash guest, less disruptive for the rebuild, DVC members appreciative to just have an option, etc etc)