first make sure the disc you are using has the ei.cfg removed. to be a all versions disk. second start the machine up with the disk. now look at the EXACT NAME of the windows versions. sometimes they are called simply "ultimate" and sometimes "windows 7 ultimate" for whatever reason some have the windows 7 added in the name and some do not. you have to make sure it is typed EXACTLY as it is here in the registry. after changing RESTART the computer. also change both the current version and the EDITION id (note the edition does not get the windows 7 added to the name. so for ultimate it would be just ultimate NOT windows 7 ultimate like the current version is) to the version you want to go to. if this still does not work change the build number to a previous build so if yours is 7100 change to 7000. normally you do not have to do this. just change the edition and the current version. also you MUST start the downgrade from within windows not from a boot up.
Why would you risk patching your system to downgrade from Ultimate to Home Premium ??? - there is absolutely no benefit.