You can change the amount of storage for your iCloud plan. Ref: Manage your iCloud storage - Apple Support
As you are aware, iCloud+ only comes with three (currently) storage plans (50GB, 200GB, or 2TB). If you currently have the 50GB plan and want to downgrade, it would revert to the non-iCloud+ 5GB plan. The storage change will take effect after your current iCloud+ subscription billing period ends. As such, you will want to copy/move anything in excess of 5GB or your data will no longer sync with iCloud.
Also, any iCloud+ features would no longer be available to you.
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