How to assign a null value to SQL paramter

I have a legacy database app that I'm working on that handles estimated effort required for projects.  The original effort for the project is defined and approved. Then as the project progresses, Change of scope is tracked as a CMF#. So existing queries used by the application find original -vs- CMF effort is done using the "Change_Index" field.

effort.change_index=null (this represents original effort)

effort.change_index= 2102 (This represents a Change Number)

So now I'm working on a function to extract either original or CMF effort and running into a brick wall and need some help. If the users selects to see a CMF, the below code works fine. But I'm having trouble calling up the orginal effort. How can I make this flexible enough to pass a Long or null value in the SQL?

Private Function ProjectPositions(ByVal iEst As Long, iDiscID As Long, iProjectid As Long, lngCMFno As Long) As Variant
    Dim EstIDTemp As Long
    Dim varPositions As Variant
    Dim rs As DAO.Recordset
    Dim strSql As String
    Dim lngcmfPram As Long
    
    Const strcProcName As String = "ProjectPositions"
    on error GoTo ErrorHandler    
    ReDim varPositions(0)
   
    If iCMFno = 0 Then
        lngcmfpram = Null  <<<<<<<<<<< This fails as you cannot assign a null value to a long
    Else
        lngcmfPram = iCMFno
    End If
    
    strSql = "SELECT DISTINCT effort.position " & _
            "FROM (tasks INNER JOIN tblProjects ON tasks.project_id = tblProjects.fldprojectindexno) " & _
            "INNER JOIN effort ON tasks.task_id = effort.task_id " & _
            "WHERE tasks.estimate_id=" & iEst & _
            " AND tasks.discipline_code=" & iDiscID & _
            " AND tasks.project_id=" & iProjectid & _
            " AND effort.change_index=" & lngcmfPram & _    '<<<< need option to make lnfcmfPram = null
            " union select sposition as position " & _
            "FROM tbl_defaultpositions " & _
            "WHERE idiscipline = " & iDiscID

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