Altering a kilt is an expensive proposition if it's done right. A standard alteration, which involves making only the apron and underapron smaller (and re-centering both) but not removing pleats is likely to cost $150 or more, if it's done by a kiltmaker who knows what he/she is doing. If you've lost so much weight that you want pleats taken out as well, it would be more expensive than that.

And, as Wally has pointed out, the fabric is cut out behind the pleats to reduce bulk, so you can't do a whole lot to repleat the kilt, even if you were willing to pay someone to take your kilt apart and then put it back together again. Much better to sell the one you have and have a new one made.