Can I transfer a balance to a card I already own instead of opening a new one?

Sometimes, but it's rarely worth it. You can only transfer a balance to a different card than the one it's on, and you can't move debt between two cards from the same issuer, which is a common restriction. If an existing card offers you a promotional balance transfer rate, you can use it, but check whether the promo is truly 0% or just a lower ongoing APR.

The reason people open a new card instead is that new-cardholder offers carry the longest 0% windows, often 15 to 21 months, while existing-card offers are usually shorter or nonexistent. Log into your current issuer's account and look for balance-transfer offers before applying elsewhere. Always confirm the transfer fee and the exact date the promo APR ends.