Hello Vlad,
There is currently no way to have the Workflow Step change based on a customer changing the Description field in the Customer Portal.
Yet, you can setup a project-level or item-level alert which is triggered whenever the Description field changes to anything. This would, of course, alert you to changes to that field whether internal or within your Portal.
Let me know if you have any further questions.
Thank you,
Tom Harder
Axosoft Support
support@axosoft.com1.800.653.0024 option 3--'Fear the Bug' Podcast--