downgraded from Iphone 3g to Original Iphone

[boyleseco]

i recently lost my iphone 3g and am downgrading back to my original iphone i had to get a new sim card and was able to get the phone to work, but when i try to restore to one of my previous back ups it says that the phone is too old and i need to update to the latest OS, and from lack of better reading i think im only able to use OS 3.1.3. Basically i dont care about anything else, but i just want my contacts, and maybe my pictures but thats it. thanks to anyone that can help