My thought. I own a couple GT’s and still have a 6.5CM. I spent a bunch of time with a 7BR years ago. It shot fine but I hated it. (Did really bad things to my elbow that the 7TCU didn’t, both in centergrip XP-100’s).
If you want a sense of adventure, go with the change. 6GT or 25GT. If change is too much trouble, stay with the BR but do hope and pray that the new barrel likes the same powders/bullets/primers that your current BR likes.
Example, our son installed a new barrel on his 6GT. His former barrel shot lights out with h4350, but the new barrel wouldn’t hit the side of a barn with that powder. Now with Varget it was spot on. New barrel, not the same. If you want to keep cases separate, you’ll probably be adding new cases, boxes, and extra time processing/reloading them all separate. It happens too. When I was shooting to 6.5’s it wound up shooting two different calibers. Chambers were so different, i used two separate loading dies, different brass, primers (small instead of large) so i used different bullets, about the only thing i used in both was the powder.
So, regardless of choice, think about them being two different calibers until they prove you wrong. (And they both use the same of everything).