Update Project budget based on forecast changes

I have a customer who has a requirement like this. 

After they setupthe project, they want to create a project budget, review it and then lock it. At the same time, they want to create a project budget forecast for this same project. Their requirement is to update the project budget based on the forecast changes. I know this is not possible because the budget is locked, and you get a warning (as seen in the attached screenshot) but I'm wondering whether there is a way to achieve this? Please advise. 

Thank you.