I've made edits and restarted the mac, and am still not seeing any changes appear.
I usually edit:
/Contents/Resources/English.lproj/AboutThisMac.Strings
found in
/System/Library/CoreServices/loginwindow.
While the file is there, and my edits are sticking in the file, they are making no difference in Yosemite while they worked fine in Mavericks.