Newer TiVo OS into Older Tivo?

So I have a dead hard drive in my S2DT, and at the same time, I was thinking of getting a bigger HDD for my Premiere.

So I'm wondering what would happen if I just took my current Premiere drive, install it into the S2, then get a new larger drive for the Premiere?

Am I playing with fire? Would there be serious problems, or irreparable damage done to hardware?