This would be a great feature request for the ideas portal on the support site. Currently, there's no trigger available for a channel changing threshold. The closest you can get is by doing a Device Event trigger on the SNMP trap for channel changes 'wlsxAPChannelChange'. Alternatively, you could also try to run RF Health Report. If the report's channel change info looks good, you can create a custom report using just that portion and run it more frequently.