Normal circular knitting, you're going from one of the size-5 needles to the other... when you switch, just use one of the size-7 needles in place of the size-5 needle you'd be knitting onto. It'll transfer itself neatly onto the other set.
If you're worried about it, you can put in a lifeline -- take a piece of yarn or thread or something and thread it through the stitches while they're still on the size-5 needles. That way, if you make a mistake, you can frog back to that point but won't have to re-do the stuff you've already done.