The railways have, over Tomb Sweeping Day (Qingming), imposed a new requirement for passengers travelling on the C trains between Guangzhou (East) and Shenzhen.
Over the break — from 03 April 2015 through to and including 06 April 2015 — you must travel on the same you are booked in. This is due to increased passenger demand. (Outside of this time period, your ticket was valid on any C train running between the two cities, especially if travelling in Second Class.)
Of course, the same applies to G trains leaving Guangzhou from Guangzhou South (Guangzhounan) Railway Station bound for Shenzhen North (Shenzhenbei) Railway Station. This rule applies to G trains all the time, as they generally assign seating for you on a particular train and are good for that train only.
Due to this new requirement, please be sure to arrive at your station in good time.