iOS 4.3 got recently released. You can do an untethered jailbreak with iOS 4.3 but there is no unlock available so far. If you rely on a software unlocked phone, it’s a bad idea to upgrade to iOS 4.3. If you still want to upgrade to iOS 4.3 and later willing to go back to iOS 4.2.1 or 4.1, you can downgrade anytime but he downgrade is only possible if you have saved the SHSH blobs to Cydia Server before upgrading to iOS 4.3. Here is how you can save the SHSH blobs before upgrading to iOS 4.3 and then revert back the upgrade.

Saving SHSH blobs

1. The tool to save SHSH blogs is TinyUmbrella. Download the latest version of TinyUmbrella
2. Run latest version of iTunes and start TinyUmbrella.
3. Connect your device.
4. Click on “Save My SHSH” and TinyUmbrella will save the fetch the signature of SHSH blobs.
5. After fetching the signature, the ticket will be saved to saurik’s server (Cydia SHSH). At this time, the “Save My SHSH” button will get disabled and will get enabled again when TinyUmbrella is done with saving your blobs.

Following is the procedure to save the host files and downgrade to iOS 4.3.

Editing and saving the hosts file in Windows

- Run Notepad as administrator by right clicking the notepad application file and clicking “Run as Administrator”.

- Go to C:/Windows/System32/drivers/etc and open the ‘hosts’ file. Add the following enter at the bottom of the file:

74.208.10.249 gs.apple.com

- Save the hosts file.

Editing and saving the hosts file in Windows

- Go to finder and click Go to folder. In go menu enter /etc in input field and click Go.

- Look for hosts file, right click it and open with Text Edit.

- Add the following entry to the bottom of the file.

74.208.10.249 gs.apple.com

-

Click on Save As and enter the name ‘hosts’ , Where: ‘desktop’, Plain Text Encoding: ‘Westren (Mac OS Roman)’ and uncheck ‘If no extension is provided, use .txt’. Click Save.
- Drag drop file to /etc folder and follow the instructions.

Downgrading

1. Connect your device to Computer and turn it off.
2. Now you have to enter your device in DFU mode. To do this, hold power and home button together for exactly 10 seconds and then release the power button but keep holding home button until USB device is recognized.
3. If you see the restore logo, that means you have not entered the DFU mode. When you enter DFU mode, the following message appears on iTunes.
   
4. Click OK and restore your device to 4.2.1 or 4.1 firmware by holding Shift Key (Windows) or Option Key (Mac), Clicking restore and browsing the 4.2.1 or 4.1 firmware file.