You can swap out the wallet.dat

Use the -rescan option when starting the client to get the program to look at your transaction record in the blockchain. This will take some time though.

I recommend using http://electrum.org/ if you intend to move wallet files around and want a portable wallet program. The official client is not designed for what you want to do.

EDIT: Also if you use electrum you can create a wallet from a seed and this means you don't have to backup your wallet file multiple times (the seed is enough to recover the entire wallet no matter how many addresses are inside that wallet)

The official client requires the wallet file to be backed up every now and then to save the address pool that is always increasing, the seeded wallets do not have this issue.