Can a Warforged add Heavy Fortification to their armor?

[Rystil Arden]

Pretty much, though quite honestly I allow similar abilities to be upgrade (e.g. minor fire resistance to major, light fort to heavy fort, etc.). At least I don't think that's a rule. Anyway, you certainly can't just swap out unalike abilities.

I actually houseruled the whole upgrade section because I didn't like that you get the full price back on some upgrades (which is why I was a bit hazy about which ones, since I don't play with the RAW on that one), but I'm definitely willing to trust you on this one

The key here, however, is that the light fort is not a magical enhancement.

Yup, absolutely.