I've changed out the entire set, when I have wanted to put a new type of strings on the uke. I've also changed out just the one string, if I'm happy with the current string set. There, how's that for indecisiveness.
Based on recommendations here, I changed to a Freemont Soloist low G. Very nice. It's also very long, so it will possibly fit two ukes.
It's great to see so many different opinions on this topic. My new tenor came with Aquila Super Nylgut strings which I am intending to keep. Which low G strings should I get to match with these?