There were no "old" versions that got erased, its a time travel movie, those personality traits of Martys family prior to his involvement in 1955 simply never existed once he became an influence on George. Marty ed and encouraged his father to stand up for himself, try new things, ask the girl out, write his stories, etc... all of which George would never have done without Martys influence. That in turn, set George on a different course in life where he would make more confident decisions and those lessons onto his children which explains why Martys brother and sister are also more successful.. no versions of anyone were ever erased, they simply chose different paths in life than they would otherwise have went, thanks to Marty.