Here's what I did on my machine, Windows 7 64-bit:
1. Download new client.
2. Exit any miners you have running.(if you have any)
3. Exit Bitcoin client (if you have it running).
4. Search your hard drive for our wallet.dat file.
5. Copy your wallet.dat file to a USB thumbdrive. (Keep this secure as backup)
6. Install new client from your download area.
7. Run new client.(verify its running ok)
8. Exit new client.
9. Search your hard drive for the new wallet.dat file.
10. Replace the new wallet.dat with the usb wallet.dat by overwriting it or deleting the new and copying old into its place.
11. Restart new client and verify addresses and amounts.
11.5 Restart miners( if you have any)
12. Profit!!! and lots of it.