How do I downgrade?
Or is my only option to outright cancel?
As it states in the info page and membership page, you cannot downgrade if it would cause a negative balance, such as "upgrading from a yearly Membership to a monthly Premium Membership" or other examples. If it's not an option on this page https://j-novel.club/upgrade
then your best option is to just cancel, and then update how you want your account after it expires.
Only other option would be to email firstname.lastname@example.org and ask for it. But this would be a decision from base to base. They surely would need to look into premium credits, if you sued them or something else.