Migrating Multiple On-Premise Companies to BC Online

ehs

Following the MS documentation for the Migrating process we have successfully migrated several customers from BC14 on Prem to BC20 Online.
https://docs.microsoft.com/en-us/dynamics365/business-central/dev-itpro/administration/migrate-data

But now we have a customer with multiple Companies in the same On Prem. Database and due to different workflows, integrations etc., they do not want to migrate all companies at the same time (there are more than 20 companies in the database).

In the documentation which I have linked to above there are two “Warnings”, which seems to contradict each other if you want to do that:

Important
If you create a sandbox environment that is a copy of the production environment, do not run the replication in the sandbox. Always run the data replication in the environment that you plan to use for production, and then create a new sandbox copy.

And

Important
Do not set up cloud migration for a production environment that is already in use for business. You risk that the migration process overwrites data that is needed to run the business. Even if your migration targets a different company in that environment, you risk that the upgrade overwrites data that is shared across companies in the target environment.

Also when completing the Data Migration from the “Cloud Migration Management” in BC Online, there is a “Run Data Upgrade” option which must be completed after the actual “Run Migration” in order to be able to start using the migrated company in BC Cloud, but if you run this option you cannot migrate other companies afterwards (you get an error saying that there are already companies in the tenant which has been upgraded).

It seems like some kind of Catch22 situation to me, making it impossible to migrate different companies over a period of time?
Or is there a hidden guide somewhere which explains how this can be done?