It's not a migration. It's not a 'alter table set in-memory'. You create new tables which are in-memory and you copy the data from the old tables to the new tables. Hence you really need downtime to do it.

I suspect the fastest way would be

Create new in-memory table

Insert into in-memory table from disk-based table

Drop table disk-based table

Rename in-memory table to the same name that the old table had and hope you did sufficient testing to ensure that there are no problems