The nature of VFP InteractiveChange event is built so that value in the control source field not changed until InteractiveChange finished. When you use debugger, all internal events finished when you stopped on some line, so value changed. In run-time this does not happen - that is a difference. Try DOEVENTS at start of the InteractiveChange event. If it not helps, check for changes just comparing this.Value and control source field value.
For correct working of OldVal you need buffering mode in alias. Otherwise you will have the same value as is current.