A change request is a must in this case. You are changing the lifecycle approach which is part of the project management plan (in the planning process you have agreed with your stakeholders to use waterfall and they all agreed to it, approved it but now it is changing...). Since the plan has already been approved and now you're in the execution stage, a CR is needed because you want to go back to the plan and change the component of the plan, the lifecycle approach.
I would ask, is it wise to change approach mid-way through a project, especially if a stakeholder is resistant to that and the other approach is already proven? It's normally better (in my experience) to decide on a method at the beginning. I'm sure you could switch, but it's just going to take a lot of communication, engagement, meetings, discussion and agreement.
What do you do when you want to change from predictive to agile but your stakeholder is resistant, because it is proven that in the past projects, a predictive life cycle is what your team has been using to succeed in the previous projects
Question 2: What do you do when your project has decided to change its life cycle, do you pause all project work ?
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