Hi,
To answer your first question, we don't have an official way to migrate Planner tasks between tenancies. However, one possibility to explore is using the Microsoft Graph API to extract your tasks from Planner and then re-ingest them into the new tenant, although this may requiring some scripting or code work on your end to make it happen.
To answer your second question - Planner doesn't currently support mentions yet, however we have plans to support this in the near future because we understand how important it is for task collaboration. We're looking to support mentions in places like the task notes, checklist, and comments, but let us know if you there are other specific places where you expect mention to work.
Thanks for your question!
