If I had wanted to merely upgrade, then all I would have had to do was restore the iPad (via iTunes) to 4.3.3. Again, that was because Apple was still signing 4.3.3 (although, I was saving blobs at that point...).
Your problem is that Apple is no longer signing 4.3.3. And, you didn't save those blobs. So, you cannot use that version, neither stock or jailbroke. Cannot.
If you want to upgrade your iOS 4.2.1, you're only option is to upgrade to iOS 4.3.5 - the most current iOS supported by Apple. Then, as Tim mentioned, you can then jail break it, should you wish, with redsn0w. However, that's only if you have an iPad1 (which you do) and it will be a tethered jail break; which means that every time you have to reboot it, you must do so with it connected to your computer.
Sorry 'bout that. Bad timing...