You can change the valve guides as well the stem seals without removing the head. Done all the time by good mechanics working to save customers money and keep their business. Expensive way is to remove the head. If the valves are not leaking compression, have him check stem play and replace the worn one along with the seals. Check the stem shims to be sure they do not have a groove worn in them that willcause side load son the valve stem and wear guides real fast.