For IDS events, there's currently no way to change the severity of events (that'd be a good idea for a feature request though). The severity is hardcoded. Typically, 'Client Associating on Wrong Channel' only appears in AMP when we see a trap come with the OID: wlsxClientAssociatingOnWrongChannel.
This particular OID is defined as:
"This trap indicates that an AP detected a client trying to associate to one of its BSSIDs on the wrong channel. This can be a sign that the BSSID is being spoofed in order to fool the client into thinking the AP is operating on another channel."
(Some more info on Aruba WIPs can be found here: http://www.arubanetworks.com/techdocs/ArubaOS_61/ROBOHELP%20UG%206.1/ArubaOS_User_Guide_-_volumes/New_WIP.htm - the portion that pertains to 'Client Associating on Wrong Channel' is under 'Detect AP Spoofing')
If you're seeing this trap fire pretty often, you may want to pay attention to how often your APs are switching channels. It could be a false positive. Do you currently have ARM enabled?