The two clients are based off of the same code base. To use Bitcoin-XT, you only need to make sure your Bitcoin Core client is shut off when you start the Bitcoin-XT client. The two programs use the same data directory and wallets, so all your coins will still be there.
See "http://bitcoin.stackexchange.com/questions/38176/bitcoin-core-or-xt" for more info regarding the differences between the two software packages.