English Version for international blogs, greetz from Italy -Google Translate sucks- (and my english too… sorry )
Today i’ll write a tutorial to “fast-swap” between CFW 3.55 and OFW 3.70, using 2 HDDs… at the end of the tutorial, you i’ll be able to swap between the firmware just flashing a dump on your PS3 using Progskeet. (instead of downgrading and loosing all data).
The first steps are not so easy, so take your time and go on, by the way, you must have some skill with Progskeet, and it must be 100% working on your console.
What we need:

PS3 Slim running with FW 3.70
2 Hard Disks
Progskeet installed and working on your PS3 Slim
Downgrade.bin edited with your personal data (there are tons of tutorials for do this)
3.55 Downgrade Dongle to do the downgrade process.
Sony OFW 3.70 UPDATE. You must login or register to view this content.
CFW 3.55 KMEAW “NO CHECK” by dospiedra. You must login or register to view this content.
Lv2Diag By Jaicrab. You must login or register to view this content.
Lv2Diag “FILE 2″ to go out of Service/Factory Mode.You must login or register to view this content.
We need 2 HDD because the NOR flashes stores some parts of the current FW on the HDD… so we will need 2 HDDs, one is for 3.55, and one is for 3.70..
We will call those HDD “A” (for 3.70) and “B” (for 3.55) dont mix them up!

Starting with a PS3 Slim with OFW 3.70.

Plug in HDD “B”, format and prepare it if it’s required by the PS3 and you should have your 3.70 up and running.
DUMP your actual NOR and call it “original dump 3.70.bin”
Now flash your “downgrade.bin” (edited with the personal data found in “original dump 3.70.bin”
Turn on your PS3 and be sure that the PS3 is asking you to press the PS button (downgrade.bin flashed correctly )
Insert a 3.55 Downgrade dongle and enter factory/service mode.
Copy Lv2Diag.self by Jaicrab and the 3.55 NO CHECK UPDATE renamed as “PS3UPDAT.PUP” in the root of your USB Stick.
Plug in the USB Stick in the most-right USB port of your PS3 and wait for it to turn OFF.
Leave the factory mode using the other Lv2Diag.self
After the reboot, you need to configure and set up your system… now you have a fully working 3.55 CFW based on KMEAW “NO CHECK”.
DUMP your actual NOR and call it “swap dump 3.55.bin”
Unplug HDD B and Plug in HDD A.
Turn on your PS3, plug in your USB Stick containing official 3.70 update and press start+select when asked.
When the PS3 reboots, check that the system is fully working and DUMP your actual NOR and call it “swap dump 3.70.bin”.
NOW IT’S OVER! You should have “swap dump 3.55.bin” and “swap dump 3.70.bin” … Now you just have to swap HDD and flash the corrispondent dump:
HDD A = swap dump 3.70.bin
HDD B = swap dump 3.55.bin

WARNING: Do not install other CFW than the “NO CHECK” one… because it’s used to make the “fast-swap” working… if you flash something different you will not be able to go between the 2 FWs. This CFW disable the Syscon checking at the startup… so it will not freeze or complain when the syscon says “3.70″ and your FW is 3.55

WARNING: In case you wanna update your console with a future “3.80″ or-so firmware. Do not update your console when you are running 3.55 firmware! You have to go to “swap dump 3.70.bin” and then update as usual (XMB or recovery)… -By the way, the downgrade is confirmed working only on 3.70… we haven’t tested it on other FWs, you could loose the possibility to go back to 3.55!-

You only mentioned Slim consoles, so does that means Phat console owners miss out?

Currently ProgSkeet only works with Slim versions as it is the only console that has NOR memory. So yes, you will miss out.

Currently ProgSkeet only works with Slim versions as it is the only console that has NOR memory. So yes, you will miss out.

Currently ProgSkeet only works with Slim versions as it is the only console that has NOR memory. So yes, you will miss out.

If you don't feel like downgrading from official firmware v3.70, but feel like enjoying homebrew on the same console, you've came to the perfect tutorial!
The well-know Italian DiGiTaLAnGeL has managed to fast-swap from Custom Firmware (3.55) and Official Firmware (3.70) with the ProgSkeet Flasher and a second Harddrive.