Easiest way is to find out what channels you're using today. Via CLI, type show ap stat t (or show dap stat t, if you have distributed APs and are on release 5.x).
There isn't any single command that will lock the APs where they are today. If I remember correctly, when you disable auto channel assignment, I believe all the APs revert to channel 1 or channel 6. You'd need to run through the config and set all the channel and power numbers manually as I mentioned in my first reply.
If you have a lot of APs, just make a text file with the commands, and chage the AP and channel number in each line, then paste that text into the CLI. Much easier than typing it out line by line.