I'm still using an iPhone 2G that I unlocked/jailbroke a thousand years ago, thanks to Hackint0sh. I haven't upgraded due to poor financial conditions.
I'm on FW Ver 3.1.3 and been on T-mobile since I bought my 2G back in 2008 (new). Baseband FW is 04.05.04_G

Recently, I wanted to buy an app, but found I needed FW 4.1 or later.

Question: is it a bad idea to upgrade my old 2G phone to FW 4.1 or 4.2 ? (jailbroken and unlocked of course)

Or maybe its not possible to upgrade my 2G beyond 3.1.3?

I should add that I turned off internet with T-Mobile, and only use Voice and Texting, and I am pay-per-month (never been on a contract with them).