This is exactly how @Marc -Devoteam- described. The only option is using Forms or using 3rd party addon. I can propose our Feature Bundle. With this app you can define when and which fields can be edited (so it can be a different set that it's on the request form). If you want to discuss your use case in details, don't hesitate to book a session with our team: calendly.com.appsvio.
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Is there a from used on the request, then in the form settings you could enable customer edit options.
If you are not using forms on requests on a portal, there are no edit options for customs, they can only comment.
Have a transition in the workflow that makes this possible, you could make this transition option available on the portal for the user to trigger or setup an automation rule to handle this based on the action taken by the user.
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