from my experience it's pretty easy if you can live with some drawbacks, like maybe changing texture formats. If everything should stay exactly the same, it can get an pretty annoying task as you have to convert e.g. the engine textures back to the right save format.. (and especially read out the mdl file before saving..)

but as already said, generally it's not too hard, and i could implement it in the next weeks (weeks not days!), as i'll work on a resave functionality..