You can't change existing control types. In addition, you shouldn't try to manipulate existing/standard data fields that have controls on the standard dialog, even if you "hide" those controls. If you want to change the way something works, then you would have to create a NEW field for it.
I don't know if that's related to the field you're saying isn't saving. If not, and assuming it's a NEW data field you created, then I would try deleting the control and re-adding it (with the quick-add fields function) to make sure the expressions get created properly.
Stan Spielbusch
[Forum Administrator]