Typically new boxes require a new service time commitment, the exception might be used Roamios as if they have been subscribed, they still have some time remaining. Occasionally TiVo will run promotions were they allow service transfers from older boxes, I have a Roamio that I purchased that they transfers the old TiVo hd plan which was $9.95 monthlly with no service commitment.
With the current monthly price rates, you will get the best deal if you prepay for a year or buy lifetime service, they used to allow you to pay a high monthly rate with no service commitment but I know I activated a used premiere last month and that was not an option (no service commitment).
TiVo Roamio Pro
TiVo Roamio Plus (3tb)
TiVo Mini (three)