Copy your wallet.dat to safe location just for sure.
Rename APPDATA\Darkcoin folder -> Dash.
Delete Darkcoin-qt.exe and copy Dash-qt.exe to Dash folder.
Rename darkcoin.conf -> dash.conf if you have one.
Update shortcut if you have one.
Done.
Haven't tried it myself yet, but think this might be what you're after. 100% make sure you have your wallet's dat file backed up before you start though.