Workflow - Field update from child to parent module

Workflow - Field update from child to parent module

I created a workflow to update a field in the "Account"-Module when a field in the "Potential"-Module is edited.
(when Potential stage is "won", change Account field "rating" to specified value).

Zoho support told me that its not possible to update fields cross-module. But in Zoho help-section is written, that its possible to update from child- to parental modules. Custom fields are not supported by now. 

Did anybody create a similar Workflow - which also worked?

thx
Bernhard