Can I update to 4.3?

You can upgrade to 4.3 anytime you want.

Now, you want to Jailbreak 4.3 that is another history, If you are on the iPad you can use Pwnage tool that is currently a tethered jailbreak, if you are on the iPad2, then you cannot currently JB.

If those are the "only" SHSH you are showing, them if you want to go back to your current firmware (4.2.1) you won't be able to, all you can go back to is 4.3 betas and 4.2GM. (base on the SHSHs you said you had)

If you want to be able to Reboot and keep your Jailbrea, you need to wait tight for abit until a untethered JB is release

 

One of the *normal* features of beta software, is that it expires, or has a time limit to it. I would not count on being able to go back to 4.3b2 or 4.3b3 when ever I choose simply because I have SHSH blobs saved for them, on your iPad1. Indeed because the use of beta software requires a special developer account, if I lacked such an account I'd be much more reluctant to put such firmware on my device...

I'm not in apples developer program, and do not know if this firmware has, or does not have an expiration date.