I think you've probably answered your own question to an extent, and newapollo has reinforced it with some more background info.
I'm going to add something here - as a bit of forewarning for if/when you call VM to ask for a box-swap. You may get a sales pitch for the new TV360 platform, which is the latest software but is very different to (and is not) TiVo. The two platforms have some substantial differences which you can read up on here if you want, but there's one critical fact to be aware of in your case...
In TiVo/V6-land, multiroom additional boxes such as yours are all fully-functional with hard drives and multiroom streaming.
In 360-land, standard multiroom additional boxes are minis that don't have hard drives. Meaning all recording is done on the master box and the mini just streams.
So... if you ask VM to upgrade your TiVo to a V6, it's just a straight box-swap and you continue as you are. If they offer, and you accept, a (voluntary) conversion to 360, then 2 things will happen....
1: Your V6 is software-converted to a 360-master, and you'll lose all recordings you have,
2: Your TiVo will be swapped for a 360-mini, and won't have a hard drive anymore.
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