I think how it works usually is that they let you use the features from higher tier plans than the one you're on; once you use them enough they send you an email asking to upgrade. That's what happened to us and I've seen other users mention it. Not sure how I felt about it, OTOH maybe it was less friction than explicitly subscribing for some "2 weeks free trial" or whatever but OTOH it did feel weird and unexpected. Anyway, we felt the extra features were worth it so ended up paying.