[AccessD] How to force an after update event in another form?

I have a form that pops up when a user clicks a button on a main form.  The 
form is a datasheet type list of (for example) Class Sections with the 
number of places available in the Class.  The user uses this form to select 
a Class Section with an empty place for a student in the main form.

I made the Class Section name control Enabled but Locked and put an 
OnDblClick event procedure on it.  When the user double clicks on a Class 
Section, the PKID for the Class Section is put onto the main form in the 
appropriate control (Form!Main_Form!cboClassSectionID = 
User_Selected_Class_SectionID_from_Popup_Form) and the popup selection form 
closes.  It works fine but it doesn't update the Class Register the way it 
does if I change the Class Section on the main form.  When I change the 
Class Section on the main form, there is an after update event which fires 
to update the Class Register.

Is there any way to force the after update event on the main form when the 
popup selection form closes or assigns the Class Section?

I've tried requerying the control and even the main form but they don't work 
because Access doesn't realize that the cboClassSectionID has been modified.

Thank you,

Penn