I'd say yes, you can buy your tickets for Yichang East - Hankou in Chongqing or any other city you'll be in during the advance ticketing period. Definitely at the train stations, and normally you should be able to buy at most large city satellite offices. An RMB 5 surcharge per ticket applies, and if you have a willing hotel do it for you, they'll add their own surcharge/service fee.