No need to download the blockchain again and you can simply replace the older version with the newer, same as always... at least for me. Never had an issue. I do keep my blockchain in a non-default directory but as with any app on osx, overwriting just the .app bundle won't have any effect on the app's associated files.

Never a bad idea to backup up your files though. If something does go wrong, it shouldn't but I'm not Nostradamus and that 5 minutes of backing up your blockchain for a quick replace if you do run into some kind of weird problems might just be the best move ever.

Short answer: No need to anything, just replace.
Good answer: Save yourself some time and have a blockchain backup - now might be a good time to start.

Hope that helps even if obvious.