I cancelled all of mine but I have a friend who cannot cancel hers. She forgot her password but the fp website did not send her a link to reset the password. So she can't cancel her account, but it might have been deactivated a few years before as she hasn't used it for a long time. Does fp email you before it bills you on this?
I have a previously-free legacy SIM, renewal date was 5/3. I did not get an email warning me of the imminent charge, but it did convert to the $3.99 Essential plan on 5/3. I'm pretty sure the change will take place on the account renewal date, not on 5/1 (unless 5/1 was your renewal date).
Chelle's suggestion sounds like the best bet-- years ago when I had issues with an account I couldn't log in to, they asked for the SIM # as verification, so you might have that ready. (I might have used the twitter support service, though.)