David,

A control will update its controlsource just before the Valid event fires. The Interactivechange is too early in the event sequence.

I would take the following approach ...

* Interactive change
...
IF < Condition to call other form >
   REPLACE (This.Controlsource) WITH THIS.Value IN JUSTSTEM(This.ControlSource)
   DO FORM < OtherForm >
ENDIF

...