The easier depends on there not being any references to the resource (e.g. from child resources). You can delete the resource from state and import it again. Importing resources uses the default provider (unless you edit the created code).

The harder option is to export the stack, find the "wrong" and default providers, edit the resource's provider from the "wrong" one to the default, and import the stack. Not harder, I suppose, but more scary, with more potential for mistakes.