How do I get my Bitcoin Core to rescan for a different wallet.dat file on OSX? I have an old wallet.dat file that I have backed up and I have already replaced the new wallet.dat, which had no btc on it with the old one, which does. I have tried to enter the command for the rescan function to adapt to the different wallet information without success. Do I do this through the QT terminal or through the debug console command line? And can this be done before the bitcoin core wallet is completely synced with the entire network? Thanks.
The QT terminal (RPC) rescan option will be available in 0.16 (not release by the time of writing). https://github.com/bitcoin/bitcoin/pull/7061

Manual rescans should generally not be required, though, if you are missing transactions in an old wallet, the rescan may help.

In Core &lt; 0.16 you need to rescan either by startup argument or by a configuration option.

Startup (OSX)

Open up "Terminal.app" and enter /Applications/Bitcoin-Qt.app/Contents/MacOS/Bitcoin-Qt --rescan

bitcoin.conf (OSX)

Place a plain text file called bitcoin.conf in ~/Library/Application Support/Bitcoin/ and make sure the textfile contains the line "rescan=1".

Make sure you remove the rescan=1 line after you have successfully rescanned your wallet.dat.