8' Is a huge amount - that would mean that the apron would extend into the pleats an extra 4" on each side. So the apron would look disproportionately wide by quite a lot. It might feel kind of funny - like sheath skirt with pleats in the back.

Having said that, I moved the buckles on one of our band kilts over 7" when one of our pipers lost a lot of weight, and you needed to look to really notice. It was an OK solution for her and wasn't noticeable in a band parade. And she gained back most of the weight anyway.

And note: if someone altered your kilt, they would only alter the apron (the pleats can't be taken out, because they've been cut out above the fell). So, if you had the kilt altered, the reverse disproportion would happen - the pleats would be 4'' too big on each side, and the apron would be 8" too small across the front. I think that would actually look odder than moving the buckles.