A lightweight access points gets its image from the wlc. There is no downgrade per say. Autonomous APs you can upgrade or downgrade. If your looking at how to upload code to access points or convert them, here are some links.
Using a TFTP Server to Return to a Previous Release
Transferring Crash file from standby: Login to the Active WLC in HA.
From CLI: (Cisco Controller) >transfer upload datatype crash (Cisco
Controller) >transfer upload filename (Cisco
Controller) >transfer upload mode tftp (Cisco Controller) >transfer
This is the start of a display filter cross reference between Wireshark
and OmniPeek. The 1st installment is a table of advanced filters. More
filters will be added as time allows. It is a living doc, so check back
for changes every so often Please feel f...