Change bucket count for Primary Key Nonclustered Hash on existing In-Memory table
As Dan Guzman mentioned in the comments, the alter isn't fully online (though should be quick enough). But another alternative that probably has the most minimal downtime is to create a copy of the table with a different name. Then use the sp_rename system stored procedure to rename the old table to a different name, and rename the new table to your old table's original name. This operation is very quick since it's basically just updating a reference in meta data on the server.