Upgrade lifetime service question

I am new to TiVo and I am considering purchasing either a Roamio or Roamio Plus and a couple minis to replace my cable boxes. I want to make sure that our family is happy with this new setup so I was thinking it might be best to do a month to month deal and if we like everything upgrade to lifetime service. Is it possible to upgrade to lifetime service at anytime or only with the first 30 days of service? Thank you