If the engine has low miles and a good service history, then you might have good results replacing just the old, age-hardened valve seals. You do not need to remove the head for just valve seal replacement. Otherwise it is probably time for a total head reconditioning, "valve job" because the valve guides and valve stems wear also. To recondition the cylinder head, it is removed, so the valve seals and valve guides can be replaced. At this time the valves and seats themselves are re-conditioned or replaced also. A mew head gasket is installed. The head can be "milled" or welded if needed. Just replacing the valve seals alone, does not require head removal. Factors to consider are the history of the engine, (miles, service records, oil usage, coolant usage, and visible exhaust smoke).Replacing the water pump, belts and hoses during this procedure is good preventative maintenance.