About hybrid public folder migration, it requires there aren't any existing public folders or public folder mailboxes in Exchange Online. So, we cannot switch hybrid to company B or C for hybrid migration.
In this situation, we need to migrate public folders to Microsoft 365 by using Outlook PST export/import.
Please note that if your public folder is large, it may take a long time to migrate:
Public folder mailbox growth in Exchange Online is managed using an auto-split feature that splits the public folder mailbox when it exceeds size quotas. Auto-split can't handle the sudden growth of public folder mailboxes when you use PST export to migrate your public folders, and you may have to wait for up to two weeks for auto-split to move the data from the primary mailbox.
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