Originally Posted by
WorldLux
This
BTW, in the past it was possible (at least in the past) to downgrade the OS if you had the file on your computer. Obviously you can only downgrade to the lowest version that runs on the phone.
That was really long ago. Apple doesn't sign IPSWs now, so you can't install anything older than 10.3.1 right now, because iTunes won't accept the file.
You can use SHSH blobs to downgrade if you have them, but it's been very hard to retrieve those and OP doesn't have a phone right now either, so his only choice would be to get a phone that's already on an old OS version.