Simple as That
Change the Popper-Fastened Buckle
Simply unsnap the poppers, take the plain buckle off the belt, and thread the tip of the belt back through the new buckle. Re-fasten both poppers, being sure that you keep the keeper in the middle of the two poppers so that you don't lose it.
Change the Bar-Pin Buckle
If the buckle does up with a bar on a pin, simply thread the tip of the belt through the bar and re-fasten both of the poppers.