Do all clients use the same wallet file? Can I just open and use the same wallet in different clients?
Or does each client have its own wallet? Which means you get locked into a client more or less?
Unfortunately, each client has its own wallet format.
If you want to try a new client, you basically have 3 options:
1. You can export the private keys from your existing wallet to import them in the new one:
Each client has support for private key export and import. However, in some clients, this needs
to be done through command-line parameters.
2. You go for the gradual transition: While you test drive the new client, you just keep using
your current one as well, and maintain 2 separate wallets
3. You decide to send the coins in your existing wallet to one of your new addresses:
This is probably the best solution once you have decided that you do want to switch clients,
but you will need to update your contacts, web pages, forum signature, etc with
your new address(es)
And of course, always back up your wallets.