I just got this old iphone4 from my aunt a few days ago and it is locked to AT&T... it has already been updated to the latest version 7.1.2 and has never been jailbroken. I have read that the only baseband that can be software unlocked is the 1.59 baseband... What i would like to ask is if it's possible to edit the iOS7.1.2 IPSW file and make it install the 1.59 baseband instead of the current 4.12.09. I was thinking of something like copying some file from the iOS4.0 IPSW file and replacing a coresponding file on the newer IPSW. I have absolutely no idea behind the engineering of these things, so I apologize if I come off as a noob.
Downgrading the baseband hasn't been possible to do on the iPhone 4 since iOS 4.0.x. Even if you had been preserving the baseband, the tool to do this with (sn0wbreeze) hasn't been updated since iOS 6, so you wouldn't be able to do it anyways. And to answer your question directly, no, it's not possible to downgrade it by injecting the older firmware into the IPSW. It kind-of used to work that way, but things have changed drastically since iOS 4 (and you needed a bootrom exploit to do it anyways on iOS 4).