Playing with gantt chart can cause a lot of problems, especially when moving releases/stages/features using mouse - it will be great to have a chance to block dates. In my opinion it should not be possible to change "scheduled" and "shipped" release dates.
As noted, the releases roadmap page currently has flexibility around the ability to schedule and organize various phases throughout the release. There is also the ability to move items around based on the schedule. With this flexibility in mind, and based on current priorities and community feedback, we do not have plans to make revisions in this area. We hope you can understand.