I've had an iPhone since the beginning, but my friend made a crack in the screen, and there were software problems, so Apple replaced it free. The replacement was a 1.1.3 OTB i believe. I was wondering if I could downgrade and Jailbreak this iPhone? If so, what loses functionality after the jailbreak?

Thanks in advance

(btw, I USE AT&T so I don't want an unlock) Would my service still work?