Thanks for the info! The first two firmwares on Bitmain's page are really old. The last one is new, but looks... "different." Like it was posted there as an afterthought. I was leery of trying it for that reason.

Meanwhile, the OpenWRT group produces new builds for the Antrouter platform constantly. Including the latest (18.06) trunk. This would presumably allow me to use the latest packages for things that aren't necessarily Antrouter-specific, like running routing protocols or other Linux-y things. If I decide to cross my fingers and take the chance to update, I'll report back. I was just hoping to hear some success stories from someone that's ahead of me on this journey :-)