I think there's value in both approaches, replacing vs. keeping. The condition of the rock and how it will impact a system over time can be affected by so many different things over the course of a few years, most of which you'll never be able to really test for in any significant way.
As a general rule I look at rock that's in systems that have never had algae problems (or haven't had them in many months/years), and have always been actively maintained to have very low dissolved nutrients as being less likely to be a source of problems. Rock that's spent time in nutrient laden waters and/or supporting lush macro algae growth are more likely to cause you issues and likely require cooking, maybe even following bleaching or a treatment with muriatic acid. If it's from a sump where there's no light, I'd look at the condition of the rocks in the tank above to try and get an idea of what sort of nutrient regime those rocks have been exposed to over time.
However, for me personally, if it was just to increase real estate for corals in my own tank and my tank was already up, running, and stable, I would probably just go with dead marco or fiji rock that had been cooked for a few weeks. Not sure why you'd need live rock from another system when the dead stuff will eventually end up as live after a few months in an already matured system. The risks of adding rock from another system unless there's a specific reason to do it outweigh any potential benefit compared to using dead rock IMO, regardless of how pristine the original system was.
If this is a new set up and this is 100% of your rock, then it's a totally different story. Getting rock from someone else's sump, assuming that sump and system in general are well maintained is probably a whole lots safer than getting rock from a fish store holding tank. You'll have an instantly cycled tank with cured rock, and you'll know the history of the rock and what it's potential issues might be. Rock from holding tanks are a total crap shoot, and I shudder to think what a test for nitrate and phosphate in one of those bins would return.
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