I can’t think of any technical issues with having a trigger set to concurrent and max threads set to 1. If you’re running a Broker client cluster (multiple IS instances using the same Broker client prefix and having the same triggers), however, this will indicate to Broker that it’s okay to hand out documents to multiple IS instances at the same time (document order=none). If serialized processing is needed, then the trigger needs to be configured as such.
AFAIK, there is no way to change from concurrent to serial without destroying the Broker client. That’s because the document order property is part of the queue management and is set at queue creation time.