You can easily change the collar if if sits right. Just copy the collar piece but redraw the outside edges the shape of a collar you have/want. (Do not forget the seam allowances)
Because you haven't changed the neck edge, it will still fit on the collar stand. You will have to move the little buttonholes of course.
If you are short of fabric, make a mockup in scrap fabric of a similar weight to check you like it.