I'm not sure I understand correctly but if you meant the ID of the current record underlying the form then try writing some code in the form's OnCurrent event procedure that changes the row source property of the combo, like this:
If Me.cboExhibitorNumber > 500 then
Me.cboClasses.RowSource = "qryShowClassesAbove2000"
Me.cboClasses.RowSource = "qryShowAllClasses"
You will either have to create the queries in question or just write out the SELECT statement. It works either way.