What happens if I buy a prepaid card during the time on which I am already a member
It adds on to your membership time. For example, if you buy 2x one month memberships (and redeem in the same month), you end up with two months of concurrent membership.
What happens if I buy a prepaid card during the time on which I am already a member
The time added from the card will alter your renewal date to reflect the additional time you've bought. You won't lose any days by redeeming it and you won't lose any deals you have in place for your renewal either (eg if you're doing the $60 year membership it will renew at that price even if the offer isn't running at the time of the renewal).