The AxesOpposed property of an InsertConstraint is a ReadOnly property, however it is possible to workaround that limitation using the “ConvertToInsertConstraint” method.
Here is the VBA code for that. It assumes that the constraint(1) is an insert:
Sub ToggleInsertAxes()
Dim doc As AssemblyDocument
Set doc = ThisApplication.ActiveDocument
Dim insert As InsertConstraint
Set insert = doc.ComponentDefinition.Constraints(1)
Call insert.ConvertToInsertConstraint( _
insert.EntityOne, _
insert.EntityTwo, _
Not insert.AxesOpposed, _
insert.Distance.value)
End Sub