MH Bulb Questions

[Beverly]

I have a 175w 6500k MH bulb over my 42g reef that's been set up for almost a year now. I seem to remember that the bulbs should be changed annually. Is this correct? If not, how often do you folks change your bulbs?

Also, I'm thinking of changing it to a 175w 10,000k bulb. Can I do that without changing the ballast? Does anyone have pics of tanks that compare 6500k and 10,000k bulbs so I can see what my tank might look like?

TIA