i upgraded to 4.0.2 the other day on iphone 4 thinking i could easily enough downgrade to 4.0.1 but i'v had nothing but problems because i never saved my blobs correctly,i think if you do it...ie follow the correct instructions u should be ok tho!
to save your shsh blobs to cydia download a programme called tiny umbrella.
when i saved my blobs they saved as 4.0.2 as well so yes it sounds right the only problem i have is itunes always comes up with an error when i try to downgrade i'v been at this for nearly 3 days now but still no further forward...i spoke to a few people who have managed it but for some reason it won't let me do it.
so its up to you m8 yes u upgrade then use umbrellas tss server to retrieve your saved blobs,
Upcoming 3.1 Exploit
If you encouter "unknown error (3002)", you probably do not have your ECID SHSH's for 3.0 "on file" with Cydia. Unfortunately, as Apple is no longer allowing users to sign the 3.0 firmware, it is no longer possible to register your device with Cydia.
Luckily, it has been reported that iPhoneOS 3.1 is vulnerable to another exploit. This means that, once a jailbreak is released for 3.1, users will be able to prepare themselves for future jailbreaks even if they missed the first round of signature storage (which I unfortunately was only able to start very late in the 3.0 game).
Once you even attempt to use this service (or if you tell Cydia to "make your life easier") you will be signed up for the signature tracker, and Cydia HQ will do its best to manage your ability to restore