Is this true? Just download the new version and then replace is that the only way to upgrade?
If you're worried (or even if you aren't) back up your wallet first. Easy peasy.
Your wallet *ought* to be in ~/Libraries/Application Support/Litecoin/
Check there and make sure the timestamps for wallet.dat are sometime around the time you closed the litecoin-qt application.
Then drag the new Litecoin client into /Applications/ and let the new one overwrite the old one. Just like any other Mac application.