We tried to use an old card because we gave my parents and MIL the number and acount PW so they could check out our photos from the photopass site while we were still on vacation. If the numbers/code have not worn, this is an option. The only problem is we were trying to use a "first generation" photopass card with a barcode and without the new square pixel code thing next to the bar code. Photographer told us the new code square works more quickly and reliably than the bar codes, so we stick to those photopasses now. Note that cards can be combined into one account when you get home so hopefully next trip we can give parents/MIL the account info again. We always end up with at least one Photopass per adult anyway because sometimes DH is at one character with DS and I'm at another with DD. Photographers will often scan a new card and hand it to you before you give them your card if you don't have it ready - we now have 4...
