Is it possible to change needle sizes in circular knitting, e.g., do the ribbing in a #8 and switch to a #10 for the rest of the project?
Changing circular needles
Yep you can do that…Just pick up the new circular needle you want to change to and start knitting like normal…this will transfer all the sts to it 
Yep, many patterns even do this, the ribbing looks neater on the smaller needles. Just begin knitting with the larger one.