impreza alarm ecu!!

Hi all, I have just been told today that my 2000 plate impreza 2l turbo (saloon) has thrown the alarm ecu and needs replaced (so said mr subaru dealer)the car fault was as follows; on starting up the left hand side indicator was lit up and would not go off until the abs solanoid fuse was pulled out. and then all indicators worked ok, the right hand side indicator and hazards on car were ok (but with no abs obviously as fuse was out)even with the abs solanoid fuse in the right hand side indicators and hazards worked fine, was just the left that had fault.on phoning mr subaru he said right away it was a tell tale sign that alarm ecu was gone. is it a easy job to replace ecu or is it as mr scooby says all secret codes to get it to run after new ecu put in? thanks very much in advance. james!! (in scotland)