Just a few of the issues I was concerned about are the collar and the button holes (number of and placement) and the pocket configuration (patch, slit w/o flap covers, slit w/ flap covers).

If the jacket has a high collar or neck line you will have a lot of extra work to re-crease the collar for one with a low neck line. Trying to mod a double breasted into a Crail just won't "look" right. The collar is different, the center line and angles are all designed to look good for a double breasted. Unless the alteration shop tears the whole coat apart it will always "look" odd.

A 5 button coat will make it difficult to put the sporran cut away in the proper place to look nice. You may end up with a button hole running right through your hem and then have to pay to have the button hole sewn shut which may or may not look right. Or you may end up with the two button holes you wanted but one is way up under your chin

A coat with no pocket flaps means you will have to use some of the fabric cut off the coat to make pocket flaps to cover the old pocket slits that will be too low in the end.

It's really more a concern of what it will take to over come the issues. ANY coat could be made into a kilt jacket. It will just be more cost effective to get a coat as close to the final appearance as you can rather than spend a lot of extra time and money totally rebuilding a coat very different than what you want it to end up looking like.