Most tub valves do not have individual shutoffs, so yes, you will need to shut off the house water supply while you have things torn out.
A valve that old won't have the mandated safety features required - anti-scald feature (required if it also serves a shower, not if a tub only).
How easy will depend on if you can access the valve without tearing things up. You will need to be able to solder, since it is almost impossible to replace one fixture with another without soldering. Having the proper tools and knowledge to be able to do that may mean buying stuff you don't have.
Some places require a permit and inspection for doing this. This is often ignored, even when required. Getting caught can be expensive if something happens.
Sometimes, you can just replace the handles and the guts of an existing valve and return it to like new condition, so that may be an option. Try to see if there is a name on it, or post a picture so the pros can help out with that aspect.