Microblogs and threads are actually different activities. It's not currently possible to migrate them, and I'm unsure of the feasibility of doing so.
To delve into ActivityPub a bit, there are "actors" (like magazines and users) and "activities" (things the actors do). A posted thread is an activity of type "Page", but a posted microblog is an activity of type "Note". They then get stored in the database of whatever instance you are on, apparently as type "Link" as far as I can tell querying my SQL database. Still trying to figure out where microblogs go and how/whether they're different, how different things are stored...
But basically to promote or migrate a microblog to a thread, somehow the actual type of the posting would have to be changed.