Cisco makes it very simple to convert an autonomous AP to lightweight mode.

Step 1- Ensure the autonomous AP is running version 12.3(7)JA or later. If not upgrade to this version.
Step 2- Download the upgrade tool and upgrade image by going to http://www.cisco.com/cisco/software/navigator.html, and selecting your product.
Step 3- Install the tool on your computer.
Step 4- Fill in the information for your AP, file location, and controller and then press the start button.

For more detailed information http://www.cisco.com/en/US/docs/wireless/access_point/conversion/lwapp/upgrade/guide/lwapnote.html

Have you ever had a Cisco access point in a remote location that you upgraded to a lightweight access point, crossed your fingers, and hoped that it associated with your WLC, only to have it fail? You could no longer telnet to the AP, issue it commands, or do anything with it? So how do you recover it?

On your Cisco layer three device, closest to the AP, you just have to enter these commands:

If you are using LWAPP enter this command

Router(config)#ip forward-protocol udp 12223

For CAPWAP use

Router(config)#ip forward-protocol udp 5246

Then under the VLAN interface it connects to enter:
Router(config-int)#ip helper-address 10.0.0.0

Where 10.0.0.0 is the address of you WLC. This will allow your AP to associate with your WLC.