Restoring from backup bypass??

ok heres the deal, i've been using ios 4.0 beta for a awhile now, and i just decided to go back to 3.1.3 because i wanted to be jailbroken again. So i backed up all my data on itunes and started looking for ways to get back to 3.1.3.

so i successfully got back to 3.1.3, and now i want to restore my data using a backup from when i was on 4.0 software. But iTunes is telling me i cant use that backup to restore my data because i am now using an older firmware on my phone. Does anyone know anyway i can bypass this because i really don't want to go back to a backup from about 4months ago because i will lose a lot of data like contacts, sms, ect... Thanks!

i am using an iPhone 3gs, and running windows vista, and windows 7

 

Thanks but the thing is, is I don't want to update to 4.0 until a jailbreak is available for it, but I really want my backup from when I was using the beta back, if theirs any hacks to force iTunes to use my backup from 4.0 on my 3.1.3?