Yearly is probably best.
You might not need to change your ballast but 10000K's do run better on pulse-start ballasts (ANSI-M137, in your case you are likely using a probe-start or an ANSI-M57).
That said, I'm running 10000K's on M57's. You should be able to get away with it, just be wary that the bulbs may be slow to start. They are a bit sluggish on the starts for me, but it's something I can live with. I'll probably eventually switch my ballasts but I'm in no real rush.
The colour of the 10000K's is a little whiter. I don't really have any comparison pictures but think of it as the difference between a warm-white and a cool-white on a fluorescent bulb. I dunno .. I like 'em. Hard to say what people will like, it varies with taste.
