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
