>does it have an impact on how often the fuel needs to be changed Yes, fissonable elements in the fuel are consumed in a manner that's proportional to output.

>does it have an impact on how often the fuel needs to be changed Yes, fissonable elements in the fuel are consumed in a manner that's proportional to output. So if you run at 25% output your fuel needs to be changed (example) every 4 years, if you run at 100% output your fuel needs to be changed every year. But it is not 100% proportional, for example, 15% of the plant's output is required simply to run the plant's accessories. So it is advantageous to run at 100% output if you can.