If it’s a recurring payment then they can cancel it on the payment provider side in which case it just won’t ever get renewed and will downgrade when the current period expires.
Otherwise, no. I’m not sure I’ve ever heard of anyone wanting to downgrade themselves after they’ve presumably paid for it unless it relates to wanting a refund in which case that would have to be triggered by you (or them but for them to do it it would be a chargeback which isn’t good).
A refund or a chargeback will downgrade automatically.
Thanks for the clarification. I can live with it. On my old website at www.wildapricot.org (club membership software) you could as a user downgrade (or better "switch") to another "upgrade". For example, if I am a "Silver" user I could upgrade anytime to a "Gold" user and improve my status then. I guess now users need to contact me if they want to upgrade their upgrade... but at least they can not break anything anymore haha.