The good news is that there are more minibus services that leave from Ao Thammachat pier, on the mainland for Ekkamai Bus Station ( 2 mins walk to the BTS (skytrain) station on Sukhumvit Rd) , in the centre of Bangkok. These depart every two hours from 0730 to 1530. Ticket price is 300 Baht.
Coming to Koh Chang from Bangkok there are still two bus 999 services at 0745 & 0945 from Ekkamai. The 0745 bus stops at Ao Thammachat and Centrepoint Pier. The 0945 bus only stops at Centrepoint but will drop people off by the road a couple of Km from Ao Thammachat - if the boat company knows you are on the bus they will pick you up from there.
Minibuses now depart from Ekkamai bus station too, leaving every two hours from 0600 to 1800 and they will drop you off at Ao Thammachat. Ticket price also 300 Baht.