Call TiVo, with the purpose to tell them you want to cancel a TiVo and upgrade to a mini. They sold me a couple minis last month for $75 each along with lifetime for $99 on one of my premieres. I kept one and sold the other.
Can't say they will offer anything similar but it's worth a try. They haven't had any sales promotions for a few montgs, perhaps you can get a good deal, if nothing else if they offer you $99 lifetime for the series 3, you can sell it afterwards and recoup the cost of the minis. Be sure to get in the cancel your TiVo phone tree, the regular csr will either say no or transfer you to retentions.
TiVo Roamio Pro
TiVo Roamio Plus (3tb)
TiVo Mini (three)