BitLocker: switch from password to USB method of encryption

I'm in the process of setting up encryption on some machines in the office, and BitLocker is the obvious choice for Windows ones. Some computers already have BitLocker activated with password protection, but our aim is to use USB keys with all of them. I can't seem to find a way of switching from password protection to USB key in a "direct" way. I'm starting to think the only option is to disable encryption and then re-enable it with USB key instead of password, but, since this process would take quite a lot of time, I thought of asking for your help first.
http://dlvr.it/ShkH5c