British Military Service: Regiments I am researching a person and on the WW1 records, it says that he was in the Royal Engineers and the Northamptonshire regiment as a second lieutenant. Is this possible, or must he have changed places.

I think he must have transferred or been transferred perhaps after his unit was merged or demobbed. We'd need more information to give you a clear answer.

If you have the service records(if they still exist), It should say what units he served in.