yes, if they have a lot of open orders then that would be a big issue. Maybe import them into the UDF?
important side note when making fields required in SO Header. Be sure to set the data source for those UDFs to populate when your updating the sales journal (SO_SalesJournal_upd) if you don't and the order has backordered lines then the backorder does not get created because the required UDF is not populated. You get no error messages or anything helpful like that either! I made that mistake on the header in the past. I would assume similar issue would happen on the detail UDF if it is required, but I have not tested it.