Same issue here. Found it last Thursday when I did a network wide update to 3.9 (3.10 seems to crash the BIOS on me randomly, but that's another story).
Here is a log of the different client CPEs we have on the network and what happened after the upgrade to the AP (indicated by the blue line). From what I can tell the CLIENTs signal dropped, not the APs. So I'm not sure if it's a negotation issue between clients and the AP or what happened. Like the topic poster I was AOK until I did this, then signals dropped like mad. Unless this is a known problem with a fix I'll be downgrading all of my APs to 2.9.51 (which worked flawlessly). The colors indicate ping response time, light green being average 30ms or less no packet loss, dark green 30ms or less and less than 5% packet loss, yellow 30ms or less and less than 10% packet loss, red is 50ms or less and 20% packet loss or less, brown is 50% packet loss, and black is 100% packet loss. I don't have signal data for before/after, but I do know all client side signals dropped about 5-15pts just by looking at it.
These are all 5.8GHZ APs running SR5s for cards. On the client side Routerboards, we still see a drop in signal but they appear to have handled the change more effectively. They also haev SR5s.