If you can repair what you have (one of the pros may know the status of that companies products), that remains grandfathered. If you replace it, it requires anti-scald technology which is usually cheaper with a single-handle valve. Those typically come on full force cold, and you continue to rotate it until it get to the temp you want. An alternative is to go with a (usually) more expensive temperature controlled unit. This usually has at least two controls - one for volume and one to set the temperature. I prefer these since you can just turn the volume on/off (and control it if that's an issue), and once you've found your ideal setting, leave it there forever, winter or summer when the incoming cold means you normally need more hot (which you'd have to manually do on a single handle unit).
If you have access from behind the valve, it's pretty straightforward. If you don't, then working through a relatively small hole means it takes longer getting the old one out and putting in a new one. If the hole needs to be enlarged (not uncommon), they sell special plates to cover it up.
From a safety and resell point of view, it makes sense to update to current codes; choosing a one or two knob control is up to your choice and budget. The time to install is the same.
Many valves come with multi-mode connections, you can either solder them on or screw on an adapter. Soldering is preferred as it's more reliable on something that is touched on a regular basis. Older ones typically came one way or the other.