I've never understood changing one string. I would only ever change the full set - one bright string amongst five, seven or nine duller ones sounds shite (IMHO!). That's my personal rule - why spend big bucks on a good instrument and then have it sound like an old wardrobe because you're too tight-arsed to buy strings?

Having said that, even though I use several tunings on one instrument, I never break strings because I change them regularly. The last one I broke was about five years ago - an Elixir 3rd, of course - broke during re-tuning! (I told you it's a rarity - so rare that I can even remember the string and the brand!) :-)

Rant over - please feel free to beat me. :-)