In order to downgrade ACS 5.x running on 1120 to ACS v4.2.0.124 then you've to reimage the ACS SE 1120 with 4.2.0.124 recovery CD. However, We will have to create our database from scratch because that database (took from 5.0) can not be restored on ACS 4.2.0.124
You must have received a Recovery CD of ACS 1120 v4.2.0.124 along with the ACS box. If you don't have a recovery CD to reimage the box, then please OPEN up a TAC case because its not available on CCO.
Here is a link to reimage the ACS 1120 with 4.2 recovery CD
Reimaging the CS ACS 1120 Hard Drive:
http://www.cisco.com/en/US/partner/docs/net_mgmt/cisco_secure_access_control_server_for_solution_engine/4.2/installation/guide/csacs_1120/Chapter_4.html#wp1045906
HTH
Rgds, Jatin
Do rate helpful posts~
~Jatin
