Perhaps someone who did this can confirm or not - but someone (I think it was Brad from Stitchkingdom, but not sure) said if you used an old card that was already attached to your account though, you lose the 30 day window to claim your photos. So in effect, you are taking a potentially 60 day window (30 days to claim + 30 days to edit) and shrinking it down to only 30 days to edit, since the photos are instantly claimed. And someone above mentioned about them being in your account right away, so I assume that's true.
So that might not work out for some people. If you had a 10 day trip, by the time you arrived home, your would only have 20 days to edit and order your first day photos before they expired.
So anyway, that's the reason I never reused old cards even though I have a bunch including the small one from the Fairy greet.