The Cisco documentation for converting an autonomous AP to lightweight also contains instructions for going from lightweight to autonomous. Download the IOS (autonomous) code for your AP model and follow these instructions very carefully:
Step 1 The static IP address of the PC on which your TFTP server software runs should be between 10.0.0.2 and 10.0.0.30.
Step 2 Make sure that the PC contains the access point image file (such as c1200-k9w7-tar.122-15.JA.tar for a 1200 series access point) in the TFTP server folder and that the TFTP server is activated.
Step 3 Set the timeout value on the TFTP server to 30 seconds.
Step 4 On the PC where the TFTP server is located, perform these steps:
a. Disable any software firewall products, such as Windows firewall, ZoneAlarm firewall, McAffee firewall, or others.
b. Ensure all Windows files are visible. From Windows Explorer, click Tools > Folder Options > View; then uncheck the Hide extensions for known file types check box.
Step 5 Rename the access point image file in the TFTP server folder to c1200-k9w7-tar.default for a 1200 series access point, c1130-k9w7-tar.default for an 1130 series access point, c1240-k9w7-tar.default for a 1240 series access point, and c1250-k9w7-tar.default for a 1250 series access point.
Step 6 Connect the PC to the access point using a Category 5 (CAT5) Ethernet cable.
Step 7 Disconnect power from the access point.
Step 8 Press and hold MODE while you reconnect power to the access point.
Step 9 Hold the MODE button until the status LED turns red (approximately 20 to 30 seconds) and then release.
Step 10 Wait until the access point reboots, as indicated by all LEDs turning green followed by the Status LED blinking green.
Step 11 After the access point reboots, reconfigure it using the GUI or the CLI.
http://www.cisco.com/en/US/docs/wireless/access_point/conversion/lwapp/upgrade/guide/lwapnote.html
