Cancel my storage plan for me now. I’ve just bought a new SE phone yesterday.

Sure, but pay attention to the following paragraph of: Downgrade or cancel your iCloud storage plan - Apple Support

"What happens when you downgrade your storage plan

If your iCloud storage exceeds your new storage plan capacity, iCloud won't sync or update your information. iCloud updates again when you increase your storage capacity or delete information to make more space. To make sure you don't lose any information, learn how to back up what's in iCloud. To help free up space, learn how to delete information that you don't need anymore.

If you downgrade or cancel your plan, the storage change takes effect after your current subscription billing period ends.* If you downgrade to the free iCloud Storage plan, you'll have 5GB of storage."