I have a 5dIII-113 and a 5dIII-133, downgraded to 123 without any problems. Hey, you can always upgrade to 133, once (if) ML becomes available. The reason it's not here yet may be, that 123 is good enough. I have one of both, just for comparison, and when I feel, that I know enough, I might downgrade all the way to 113. The up/downgrade doesn't disturb me, since I'm convinced it won't interfere with product warranty.
I just turned 63, and when I can do it, you must accept, that the process is very much foolproof.