Never use an update query on a record your form is bound to. I'm also pretty sure that how forms handle bound RecordSources has changed to be more like the way Reports do. In that case, the field you want to update with VBA must actually be bound to a control. You may make it very small and hidden. Then in the BeforeUpdate event of the form, update the hidden control:
Me.ChangedDT = Now()
Me.ChangedBy Environ("UserName")