I think the simplest way at the moment would be to keep your bitcoin-qt installed after you install your new client then create a new wallet with the new client and send all your BTC (perhaps requiring a tx fee which really will be most likely negligible) to the new address in the new client.
Oops - I should probably have said "send several tx's" with the old funds (the first being just a test one to make sure the new software and wallet is working).
A side benefit from sending the old coins to multiple new addresses is that future tx's will have a better chance of avoiding tx fees due to "coin age".