Also you need a lock the suits the door knob, see my web page which will help describe this. www.handlesupplie.ie/doorknobs Suffice to say you need the knobs to be a least four inches back form the leading edge of the door. This is in order to make sure your knuckles are not rapped by catching them on the door frame when entering the room.
You can of course replace your present locks with four inch locks and fill the openings left by the old locks with suitable wood fillers and these can then be sanded and painted over. In fact I recently supplied new doors knobs to a customer who kept her existing doors and changed the door locks from standard 2.5 inch vertical locks to four inch vertical locks to accomadate the knobs.