Well, no update.
It seems that because phpBB 3.0 is at end of life, we really
should go to 3.1.9. However, there are numerous incompatibilities between 3.0 and 3.1. Some are simply additional risks in making the conversion (like database schema updates) and others are a loss of functionality (that fortunately won't affect many of us here, like losing some themes).
Anyway, this means more testing on my part as well as rounding up some stats on just how much we'll lose when we switch. We might lose nothing (maybe no one here is using the themes that we will have to give up?) or it could be something significant (will I need to recode the spoiler support? how long will that take?).
In any case, it's too detailed to make the change right now. Since Real Life is going to get in my way over the next few weeks, I expect to do some testing and simulated conversions, but not throw the switch until the week of August 1st. I'm targeting 1:00 AM EDT (5:00 AM UTC) on Wednesday, August 3rd.
I apologize for the hassle, and I'll update this thread again when I'm closer to making the switch. This thread will also go back into the "global announcement" status when I know more. Thanks for reading this far!