You generally can't undo ribbing from the cast on because of the knits and purls, though it may work in the round. Try it and see if it works. If you just get a tangled mess, then snip a st where it changes from the rib to the stockinette, put the sts on a needle and knit a row of stockinette, then redo the rib from there down. You'll have to add a stitch because going the direction you're offset by one stitch.
sue- knitting heretic