You can always add a little velcro to the kilt (can get it at any fabric store) to make it fit again. Anyone you know with a sewing machine should be able to add it (or at VERY least if you take it to a dry cleaners with a "taylor service" they will add it for $10 or $20).

The problem will be when you lose a LOT of weight... the front apron that used to go from "hip to hip" will then go from one "cheek" around the front to your other "Cheek". In that case, you'll have to re-do the front apron or pony up the funds fo a new kilt.