You can swap them but you have to do a Clear and Delete. You can't view the content from one box on another by swapping the drives since each box has different encryption keys.
thanks for the info before I swapped it yesterday night...To reiterate: no. When/if you exchange hard drives between like model Tivos, the only way they will operate(properly) is by doing a "Clear and Delete Everything" from the menu upon start up.
This essentially reverts the Tivo back to the state when it was new. Virtually erases everything. Everything must be started from the beginning. Guided set up must be repeated, cable cards must be re-paired with the cable company, etc.
However, it may be possible to restore your seasons passes, as they are stored on the Tivo corp servers. I am not exactly sure how this works though. Log in to your account at tivo.com and go to the "Season Pass Manager"(or something similarly named) to possibly gain a better understanding of how this may work.
But outside of transferring any shows you want to save to a computer, and later transferring back after the hard drive exchange, there is no other way to save recordings.