newapollo has beaten me to a response, but I will still wade in here.. And sorry for the length of the post, what I'm saying can't be said succinctly, unfortunately.
The short answer is, yes you can replace your TiVo with a V6 or 360 - but please, be very careful what you ask for. The two boxes are the same hardware, but the software they run on is substantially different - and 360 has one particular difference you need to be aware of. You also need to have VM broadband in order to swap - that's a requirement.
If you swap the TiVo for a V6, that's a straight change to a newer box running the same software. Job done.
360 is a completely different software to TiVo with some very substantial differences. There's 1001 threads on here comparing them, and opinions vary wildly, but one notable thread worth a read is here - and you should be aware, a conversion to 360 is one-way with no option to revert if you don't like it.
You should also be aware: In TiVo/V6-land, multiroom boxes all have their own hard drives with full streaming between the two. In 360-land, multiroom boxes are minis without hard drives of their own - they slave from the master box.
So... if you do decide to convert to 360 and do it in "one go", your existing V6 will be software-converted to 360, and the TiVo is swapped for a mini-box thus losing its storage space. You are much better off upgrading your second TiVo for a V6 and retain the HDD.
If you then subsequently decide to convert to 360, then both your V6 will be software-converted and retain hard drives.
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