It turned out to be a piece of cake....once I knew what I was trying to do. I thought I was supposed to take bonnet off, then unscrew actuator. I didn't know that "unscrewing the actuator" meant unscrewing whole top of valve without removing bonnet first. Then it unscrewed easily, no wrenches needed, there was no glue, I screwed in top of new valve, and it was a five minute operation. Well, I stopped to take pictures, and also had to remove the solenoid of valve next door to make room to unscrew bad valve's actuator, so more than five minutes, but whoo-hoo, no need to cut pipe. Thanks to Wet Boots and Mr Fix-it for your help.