Azure Information Protection December update moves to general availability

Working with a customer currently and I'd like to get more clarity over custom permissions. When testing, we set a custom permission on a file so one user could open the file. We sent the file to 3 users and as expected, only 1 was able to open it. Then we updated the perms and granted the other 2 users access and saved the file. The two users could not open the file until we sent them the updated saved copy. How can perms be updated without having to redistribute the file? I was under the impression that the perms were stored on the RMS server, not on the file itself. This is a big deal breaker. Having to manage multiple copies of the same file floating around with different permission sets is a nightmare for users.