Their policy states (
http://www.sgccard.com/Guarantee.aspx):
If the grade determined under such review is lower than that originally assigned to the card, SGC shall, at SGC's discretion, either replace the card or pay the difference between the current fair market value of the card at the newly established grade and the current fair market value of the grade originally assigned to such card, in the form of either cash or grading credit.
At least you would have a little protection if it went down a grade, though "store credit" for more grades that might also go down is a pretty crappy guarantee.