If the table is empty, you can alter the type of a table from an in-memory table to a disk-based table (or a disk-based table to an in-memory table) by using the ALTER TABLE statement with the STORE MEMORY or STORE DISK clause, see ALTER TABLE:

If the table contains data, you must create a new table with a different name to which you copy the data. After copying the data to the new table, you can drop the old table and rename the new table with the same name as the original table.