Code says you can repair an old valve, but if you replace it, it must meet current codes, so yes, it sounds like you will be replacing the thing. To give you enough room, in case you can't get into the wall from behind, they make what are called 'remodel' plates that would cover the bigger hole you'd have to make to gain access to be able to get the old valve out and solder in a new one. Code requires all new shower valves to have anti-scald technology. In most cases, this a a single handle valve. The easiest way to handle the diverter is to replace the tub spout with one that has a diverter built-in. The one that I've tried, and others seem to like, is made by Delta and comes in various models and finishes. Look for a 'pull-down' diverter tub spout on the Delta site. Delta makes the remodel plate (which I've used), and almost every other manufacturer in various styles and finishes. Basically, you use the template to make a bigger hole, tear out the existing valve, install a new one, install the remodel plate, then put the valve's trim on. Other than replacing the tub spout to provide the diverter function, you're done.