The reason I said it would impact duration is the assumption that the previous version and new version of the system are all in the code at one time, managed via feature flags or something. I think I was picturing that due to other comments later in the thread, you may not be handling it that way.

Either way, I still don't understand how you can reasonably manage the complexity, or what value it brings.

Example:

main - current production - always matches exactly what is being executed in production, no differences allowed

production_qa - for testing production changes independent of the big project

production_dev_branches - for developing production changes during big project

big_project_qa_branch - tons of changes, currently being used to qa all of the interactions with this system as well as integrations to multiple other systems internal and external

big_project_dev_branches - as these get finalized and ready for qa they move to qa

Questions:

When production changes and project changes are in direct conflict, how can you possibly handle that if everyone is just committing to one branch?

How do you create a clean QA image for all of the different types of testing and ultimately business training that will need to happen for the project?