Good question, we decided to not allow a flow type change via the UI as the implications of this change are far-reaching, but more for a change from product / waste to elementary flow, not vice versa. You can change it via sql, in the sql window:
update tbl_flows set flow_type = 'PRODUCT_FLOW' where ref_id = 'and_here_the_UUID_of_the_flow';
Or WASTE_FLOW if you want to change to waste.
