Access vba modify query def to change columns displayed?

You can't requery, you have to refresh the subform's source object:

MySubformControl.SourceObject = ""
MySubformControl.SourceObject = "Query.MyQuery"

For testing, I created a table Table1 with fields Field1,...,Field4, then a form with 4 check boxes and a subform, then a query Query1 which the fields from Table1. Here is the code behind the form (I let Access name all my objects, so the subform is called Child8):

Private Sub Check0_AfterUpdate()
    Rewrite_Query
End Sub

Private Sub Check2_AfterUpdate()
    Rewrite_Query
End Sub

Private Sub Check4_AfterUpdate()
    Rewrite_Query
End Sub

Private Sub Check6_AfterUpdate()
    Rewrite_Query
End Sub

Private Sub Rewrite_Query()

    Dim qdf     As QueryDef
    Dim strSQL  As String

    Set qdf = CurrentDb.QueryDefs("Query1")
    If Check0.Value = True Then
        If Len(strSQL) > 0 Then strSQL = strSQL & ","
        strSQL = strSQL & "Field1"
    End If

    If Check2.Value = True Then
        If Len(strSQL) > 0 Then strSQL = strSQL & ","
        strSQL = strSQL & "Field2"
    End If

    If Check4.Value = True Then
        If Len(strSQL) > 0 Then strSQL = strSQL & ","
        strSQL = strSQL & "Field3"
    End If

    If Check6.Value = True Then
        If Len(strSQL) > 0 Then strSQL = strSQL & ","
        strSQL = strSQL & "Field4"
    End If

    strSQL = "SELECT " & strSQL & " FROM Table1"

    qdf.SQL = strSQL
    qdf.Close
    Set qdf = Nothing

    Child8.SourceObject = ""
    Child8.SourceObject = "Query.Query1"

End Sub