Pretty sure you should do the master first, then proceed with the locals.
To the best of my knowledge, having locals at a higher level the master will prevent them from receiving any sort of updates, but that may only be when jumping between codes (eg: 5.x to 6.x).
I had my network at 5.0.3.3 and upgraded a few controllers to 6.1.x for testing. I had to change my RADIUS server IP and the only controllers that didn't receive that change via an update were the ones running 6.1.x.
I believe "simultaneous" may not be meant to be taken literally. You should be okay to upgrade within a relatively short time period - my rep told me a few days would be more than fine....with the above caveat that changes may not work until all are running same code.