I concur on the change tasks in the ECN. You can also create ECN task templates so these tasks are generated automatically on creation of the ECN plan - you would just have to fill out the responsible party and the reviewers. Also, you can set up policies so certain roles can modify objects at any state. So say you have a role "Planner" (an ootb role, or a custom role that has been created by rebuilding the rbinfo files).
At either the Site, Org, or context level (determine which is appropriate for your scenario) level, you create a policy that grants the role "Planner" to "Modify" & "Modify Content" at the state of "Under Review".
Now, some caveats. The lowest level approve or deny applies, and this gets important if a person serves in multiple roles. Say you have a "Product Manager" who also serves the "Planner" Role. And at the Org (or site) level, you grant "Modify, Modify Content" to the Planner, and at the product level, there is a policy that says the Product Manager has a deny or explicit deny to modify anything at "Under Review" - then the planner would not be able to make changes.
I would map out how you want your process to work, your lifecycle - and how you want to interact with what data, at what states, then decide if you want to have a company policy to restrict people from serving multiple roles, or to define certain role permissions at a higher level so that your grant happens at a lower level and will override the deny.
Once you have this mapped, then you can get an idea on how the lifecycles & policies need to be set up, and guidance to team members can be given so they know what results to expect.
