Create a copy of an existing shared field with copy/paste.If the existing shared Field was named XYZ the copy is named XYZ_1. Select the new field and press F2 to give it its proper name: for instance ABC.
Then open the new shared field and make the changes to its @Formulas. Save it.
Use the new shared field in a form, then test the form. You get an error message like “Shared Field not found: ABC”
The thing is, the field ist displayed with the new name ABC, but ist still referred as XYZ_1. If you reopen the field in designer and open the property box, the field has still the name XYZ_1. Change it here and everything is fine.