Migrating content down a level in wagtail streamfield

I've got a wagtail page model with an existing streamfield. The streamfield has now changed format and I'd like the text content to migrated down a level into a streamblock.

The initial model (simplified):

class ProjectPage(Page):
    description = StreamField(
        [
            ("text", blocks.RichTextBlock()),
            ("image", ImageBlock()),
        ]
    )

The new model (also simplified):

class ProjectPage(Page):
    description = StreamField(
        [
            (
                "section",
                blocks.StreamBlock(
                    [
                        ("text", blocks.RichTextBlock()),
                        ("image", ImageBlock()),
                    ]
                ),
            )
        ],
    )

Currently, the image blocks are empty, so only the text item needs migrating.

I tried looping through pages and doing old_data = page.description.raw_data then using that to make the new content [{'type': 'section', 'value': old_data}].

If I page.save() and get the value for page.description.raw_data, I see the new content, but the admin still shows the old values.

I'm guessing this is linked to revisions vs published versions but I can't figure out how to fix this.

Any pointers would be welcome!