I will do my ribbing using, for example, an interchangeable US6 circs. Then when it's time to up the needle to 'gauge size' US8...I'll change the right needle tip to the US8 for the first row. However, sometimes I'll knit a few rows without changing the OTHER TIP to the US8, essentially knitting with a US6 at one end and US8 at the other end!
Eunny Jang, from Interweave Knits and Knitting Daily TV says, "not to worry....the gauge with level itself out to the US8. No need to frog back those few rows....the US8 will expand the US6 stitches to meet US8 gauge."