Q: How do you change an iTunes gift card  from one account to another?

You Cannot shift the iTunes Balance from one Account to another, once the Gift Card is redeemed with one Account but you can Gift some contents from iTunes to someone by following the method mentioned in the link below.

http://support.apple.com/kb/HT2736

Click here and ask the iTunes Store staff to put the balance back onto the card; they'll only do this if none of it has been spent.

iTunes Store credit can't be spent on any sort of gift.