So, I first started mining over a year ago but I cannot get the QT Bitcoin wallet to launch when I swap out the new wallet for the old (pretty sure I know what I'm doing there). It gives me a database error.
The debug.log will show the specific error, but if your blockchain was caught up and all you are doing is replacing the wallet.dat with an older one from a backup and the client won't restart after that then the problem is likely difficulty in accessing that wallet.
The newer clients perform more strict checking and a wallet.dat that was useable in a previous release can become unreadable with the most current release. [Clarificatoin: If it already had problems just that the old version was oblivious to them.]
Make sure that you keep another copy of the old wallet.dat intact and are only working with a copy of it.
What some people have tried is going back to an old version of the software (e.g., v0.5.x, or earlier even) and try accessing the wallet.dat with that.
You can also try using PyWallet or Gavin's BitcoinTools to access and export the keys:
For linux, ./dbdump.py --wallet
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https://github.com/gavinandresen/bitcointools
