As for proactive measures, you can take a working Tivo hard drive (S3 OLED or TivoHD) and make a truncated backup of the Tivo software that you can later restore to a new hard drive.
If you have a Windows PC, then you can use WinMFS. Otherwise you can use the Linux based MFS Live CD.
Both available here:
As for swapping hard drives, yes you can do this.
It only works if the model numbers match (you can't swap an S3 OLED hard drive into a TivoHD or vice-versa), and you have to perform a Clear & Delete Everything to "marry" the hard drive to the motherboard.