Using an already downloaded IPSW file for updating (not restoring!) an iPhone?

[illjazz]

I had to jailbreak/unlock my iPhone 3G when updating to 4.0 to use it. Because of this, I already have the original 4.0 IPSW file.

I'd now like to update my girl friend's 3GS, which does not need to be jailbroken because she's using it with the mobile carrier she bought it from.

We're in a location where we don't have wi-fi access and I'm using tethering to post this. Since I already have the IPSW file for 4.0, I'm simply wondering if there is a way to force iTunes to recognize this fact? I've already placed the IPSW in the Library/iTunes/iPhone Software Updates/ folder. iTunes doesn't seem to care and just starts downloading the update file from scratch.

Is there a way to let it know that the file is already downloaded and ready to use for the update process?

I could just do a restore, of course.. but I want to avoid losing the SMS history, app settings, etc.

TIA

 

[benflick]

Hit Shift+check for updates (Windows) or Option=check for updates (Mac), then select the IPSW you previously downloaded.
Actually, you will need the 3GS IPSW for the 3GS, the 3G IPSW and the 3GS are incompatible.