If I purchase life time Infinibee today, will I be able to upgrade to life time Bee Plus in the future by paying the difference?
Affirmative! Quoting the FAQ on beeminder.com/premium:
Q: What if I get lifetime Infinibee but then decide to upgrade?
The full cost of lifetime Infinibee is subtracted from whatever you upgrade to. Here’s the general principle we’re going for: You can jump on any plan with any payment frequency at any time. If you change your mind and upgrade or downgrade you won’t have wasted any money by doing so.
The only way to regret any premium purchase is to buy more than you actually wanted, like buying a year of Bee Plus when you only wanted a month of it. Or to buy Beemium when Bee Plus would’ve sufficed. Or buying lifetime and then dying within 7 years. (Technically you could replace “dying” with, say, solving your akrasia for good or something.) If any of those things happen, talk to us and we’ll make sure you (or your heirs) are happy.