since both are a charge card, if I'm not mistaken, it wouldn't really make any change on your credit score. but it may offer additional rewards with AMEX.
But how would you know...these are not supposed to have pre-set limits anyway?
No limit appears on your credit report, but AMEX has a secret limit... but since no limit is reported to the CRA's... changing the type of card won't really have any effect what so ever on your file..