"BitCoin-QT" Core Upgrade - PRIOR Steps?
bitcoin-qt 0.8 changed the database structure. Bitcoin Core 0.15 changed the database structure again. There is no direct database structure upgrade path from bitcoin-qt 0.7.x to Bitcoin Core 0.15. You will have to upgrade to a version between 0.8 and 0.14 first before you can upgrade to Bitcoin Core 0.15. I recommend first updating to Bitcoin Core 0.14.2, letting it build the databases and sync, then upgrading to Bitcoin Core 0.15.0.1.

See also: https://github.com/bitcoin/bitcoin/blob/master/doc/release-notes/release-notes-0.15.0.md#how-to-upgrade