Nylo, are you thinking about doing that yourself? If so, it wouldn't be too hard to move the buckles and straps. Is your kilt pleated to the sett or to the stripe? The apron would overlap the pleats by 3", which would not be bad if it's pleated to the stripe. If it's to the sett, it would throw off the look of the back because the tartan pattern would no longer be centered.
The other way would be to have a kiltmaker alter the kilt.
This is what I would do:
.Carefully take out the stitches for the lining, top band, apron canvas, edge of first and last pleat along aprons, apron facings and fringe. Also unstitch the straps.
.Unstitch the little curved hems on the deep pleat of the apron and underapron.
.Steam press the old creases to make the fabric smooth.
.Measure and mark the new apron and underapron with the revised measurements to make the aprons narrower on each side by 1.5", making the kilt 3" smaller.
.Pin, baste and press the new apron edges.
.Hand sew the the inner apron edge to the first pleat and sew the last pleat to the underapron edge.
.Sew the fringe edge of the apron. Sew the facings for the apron and underapron.
.Sew the little curved hem for the deep pleats. Press the deep pleats.
.Sew on the straps, canvas, top band, and lining.

It definitely takes some time to do this, but it's not so hard for a kiltmaker to do.
.