How to migrate from StaticBlock to StructBlock?

I have to change an already existing StaticBlock to a StructBlock:

class SomeBlock(blocks.StaticBlock):
    pass

    class Meta:
        ...

to:

class SomeBlock(blocks.StructBlock):
    ...

    class Meta:
        ...

However if the wagtail page has already SomeBlock configured in it, I receive the error:

NoneType is not iterable

Since I don't have anything inside the StaticBlock. I need to write a custom data migration for this.

Based on Schema Operations listed, I couldn't find a way to change the actual block type. How do I approach this?

StructBlocks and StaticBlocks live inside StreamFields so you need a Wagtail-specific method for converting blocks within a StreamField. Please see https://wagtail.org/blog/google-summer-of-code-toolkit-for-streamfield-data-migrations-in-wagtail/ for a discussion of the issue. Then you can use the project described https://github.com/wagtail/wagtail-streamfield-migration-toolkit Or, if you want to try out Wagtail 4.2rc1, you can try the built in migration helpers: https://docs.wagtail.org/en/latest/advanced_topics/streamfield_migrations.html#streamfield-data-migrations