Reference old value in a workflow
I’m not sure whether you can source it (as in fill in another field value from it). I’ve only ever seen it used in the workflow conditions. You could try building it in the conditions, switching to ‘custom formula’ and copying that into the Formula parameters and see if that works, but it’s not selectable in the From Field parameter dropdowns, which makes me question whether that would work. Alternatively, depending on your use case, you could just throw an error before submit when xxxx field value is empty, and tell them in the error they’re not allowed to do that.
Hope that helps! Let me know if the formula thing worked.