The license change won’t apply retroactively - I am not sure theres a legal way to retroactively change licenses and terms? I am recalling back to the Unity runtime fee, which they wanted to apply retroactively, but there was a lot of noise/discussion on whether it was legal to even do this.
OK, in that case it may not even make much sense to add a license. There will be no added code to this repo in the future, so there will nothing the new license would apply to.
Once you have main released version of the repo that contains the license you want to use going forward, any branches from that point should contain license by default? Since its just a file in the main branch.
Yes, you kind of answered this in question 1. Since it is not retroactively applied, it won't apply to the stale branches that only exist as snapshots of the code.
Since you are using it commercially, and want to change the license for future versions, you will absolutely want to discuss this with whatever entity is using it. You could choose a license they refuse to accept, and end up not being able to use any future releases. My employer will not use copy-left style licenses for example.
Good point. This is not included in any software that is distributed, it is only a smaller part of an internal codebase used for data analysis. Does that not change things? But to be on the safe side, it would probably make sense to make it as permissive as possible to avoid any issues here. But then again, if it is not applied retroactively then nothing of the code used will be subject to any license. But good thing to remember for the future.