How to compare a form's OLD and NEW values of a field?

We want to compare a form field's OLD and NEW values.
So that OnValidate or OnSuccess, we can trigger a costly routine *only* if it has changed.

But we can't seem to refer to the "old" value. We placed it into a Deluge variable during On Load. But, the var is no longer defined in OnEdit or OnSuccess.

Something tells me that I'll need to create a special field for it's old value, and store it there On Load.
Then, refer to it for the comparison...?

Advice appreciated! Thank you!