Normally, you don't upgrade to LWAPP by replacing the image and rebooting. You should use Cisco's upgrade tool to do this conversion, which can be downloaded from the wireless downloads site.

As for reverting, you can perform the following:

1. Pull the AP down and connect your laptop directly to its ethernet port (using a crossover if needed). Your laptop needs a TFTP server installed.

2. Assign your laptop an address in the range of 10.0.0.2 and 10.0.0.30

3. Open your TFTP server and place an autonomous image in the root directory. Rename it "cXXXX-k9w7.tar.default", where XXXX = the AP model number (1100, 1130, 1200, 1230, 1240).

4. Hold the MODE button down and power up the AP. Hold it for 30 seconds or so until the light turns red. Let it go and it will download the code.

http://www.cisco.com/en/US/docs/wireless/controller/4.2/configuration/guide/c42lwap.html#wp1105215