Can I upgrade 3.1.3 to 4.0.1? 3G and 3GS

[keyztolope]

I have 2 iphones, a 3G and a 3GS that I want to sell.
They are both jailbroken on 3.1.3
I want to upgrade them to 4.0.1 to jailbreak/unlock to sell for max profit

Is this possible? Am i required to upgrade to 4.0.2 then downgrade?
Any help would be greatly appreciated, thanks!

EDIT***
For the 3G i have read that it is best to keep 3.1.3
How can I do a clean install of 3.1.3 on the 3G and jailbreak/unlock?
It currently has 3.1.3 on it now but is loaded with personal info/crap apps/etc

[Timtacious]

If you open Cydia, what do the phones say at the top? They should say "Cydia has these SHSHs on file:" and then list firmwares. You can only upgrade the 3GS if you have 4.0 or 4.0.1 blobs.

You should be able to update the 3G to 4.0.1 with no trouble, though.

How To Downgrade from iOS 4.0.2 to iOS 4.0.1/4.0 for iPhone 4/3GS/3G/iPod | Mobile-Geeks

[Trooper Sam]

But in the absence of a desire to upgrade the 3G, all you need do is download a copy of the 3.1.3 firmware, crank up iTunes, connect the 3G, shift-click the restore button, and select the 3.1.3 firmware you downloaded. It'll be a pretty complete wipe, not like NSA good, but it'd take a pretty determined guy to get at any personal info.