I followed some advice on several of the posts regarding this subject. I have a 3GS on ios 6.2.2 and ipd 4 now on ios 9.0.2. After originally, making the mistake of updating the ipad to ios 9 and finding out my iOS 8.4 update was not to date, I restored the ios 8.4 to the iPad and of course lost some data.
http://www.macworld.co.uk/how-to/iosapps/how-downgrade-from-ios-9-to-8-7-reinsta ll-ios-8-3522302/?p=2
Backing out the ios 8.4 using a complete restore, using the file per https://ipsw.me/ file for my ipad. Worked perfectly.
Then restored my IOS 8.4 back up to the ipad. Cleared the music and movies to make sure i have enough room. Sync o iTunes.
Create an archive of the back-up, by right mouse click archive from the iTunes Device prefenence window.
Additionally, find that file - Right Mouse open in finder. Then Right Mouse and compress it. A .zip file will be created.
From there. I used over the Over The Air updated to get to ios 9.0.2. Then Wireless back-up to my iTunes 11.4 on OS 10.6.8
Haven't done the wired update yet. Don't want to break it.
My iphone still updates with the cable and without. ( it is on a different account though ).
Hope this helps.