Plone 5.2 upgrade: ImportError: No module named contentmigration.basemigrator.migrator

I am upgrading from 5.1.6 to 5.2-latest, and I encounter the following issue.

ImportError: No module named contentmigration.basemigrator.migrator

This is because the plone.app.contenttypes dexterity migrator depends on Archetypes, but Plone 5.2 doesn't ship with it any more. I have read through the below items, but I wanted to be sure that I understood the resolution correctly. It didn't seem to have a 100% complete solution that I could tell, but forum posts do present some potential working solutions.

See: Upgrade to 5.2 failing with 'IATCTTool' has no attribute '__iro__'
See: https://github.com/plone/plone.app.contenttypes/issues/99

During an earlier migration from Plone 4.3 to Plone 5.0.6, I have already migrated from Archetypes to Dexterity. If I read the steps properly in the posts which I mention above, it says to roll back from 5.2 to 5.1 so that I can run the GenericSetup uninstall profile for Products.ATContentTypes. Then I should be able to resume the move from 5.1 to 5.2.

If anybody could verify that, I would appreciate it greatly. Thank you!

plone.app.contenttypes includes the migration package only if Products.ATContentTypes and archetypes.schemaextender is available (see https://github.com/plone/plone.app.contenttypes/blob/master/plone/app/contenttypes/configure.zcml#L39). Otherwise another package depends on Products.contentmigration

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