Changing/Manipulate the value of an access policy session variable

Try this:

when ACCESS_POLICY_AGENT_EVENT {    
   if { [ACCESS::policy agent_id] eq "change_username" } {
   Cut/Change session variable to correct value
   set new_username [string trimleft [ACCESS::session data get "session.logon.last.username"] "ras_"]
   ACCESS::session data set "session.logon.custom.username"] new_username
}
}