If this is a kilt built the traditional way the buckles is not sewn directly into the fabric of the kilt. If you look carefully at this picture you see that the buckle is connected to a strap of fabric similar to the kilt fabric making it almost invisible:

(courtesy of English Bloke)

With a litle surgical knife or scissors work you can remove this buckle without damaging the kilt, and sew it back on again where you want it. The buttonhole on the left side can be a challange because the edge of the inner apron will stop you from pulling the strap further trough the hole. You can solve this by moving the strap from the edge of the apron to the required posision further to the right on the apron. A second option is to close the button hole, sew the left hand buckle to the inner apron edge and sew the strap to the inside of the kilt, like this:





Skål!