Update 3GS with iOS3 to iOS4?

[vkping]

Greetings!

We purchased an iPhone 3GS to test our iOS software on an older device.
The 3GS has currently iOS 3.1.3 installed.
I would like to upgrade it to iOS 4.2.1 since this is the minimum version we support. I found the correct .ipsw, but restoring / upgrading fails with the message "this device isn't eligible for the requested build".
Apparently there is some sort of code signing in place which prevents my device from being upgraded to anything other than iOS5.1.

I googled a while, but while that error message is pretty common, I couldn't find a solution to my particular problem: To restore a device from iOS3 to iOS4 while there is an iOS5 update available.

Is there any way to "officially" install iOS4 on the device? I don't think that jailbreaking would be a solution for us, we would like to have a device which is as close to a typical iOS4 user as possible...

 

[DerekRod]

If apple is no longer signing iOS 4.2.1 it is impossible unless you have saved SHSH blobs for that device

Edit:They are still signing it but only for the iPhone 3G

 

[goosnarrggh]

If you're a registered developer, perhaps you may find some help contacting the Apple Developer Connection technical support.

I suspect, though, that their first recommendation would be that you should focus any new development effort only on the newest iOS available for each generation of device you intend to support.

 

[alphaod]

Edit:They are still signing it but only for the iPhone 3G

Last time I checked they were still signing for the 3GS as well.