Since one half of the boxing world ranks Joe as the no1, the other half as the no2 p4p fighter, depending on his win, he could get a top5 ranking...
Roy would be on a similar status as Jesus, and Calzaghe's legacy would be ruined imo, he hasn't beaten enough elite fighters to lose to a past prime fighter. Though it would depend if he bounced back, if he beat say Pavlik after it than he would still go out in great fashion.
a lot of people now had RJJ in their 20s or 30s in the ATG list... i'd say he moves to top 15, or perhaps i would still re-assess and take another look.
Only way this happen is if Joe C. turns old overnight like morales pac 2. Roy is way past his prime and I cant see how he would beat Joe let alone KO him.