I'm in the process of setting up a trip for May myself and I'm reading mixed things about Chamexpress. From the very good to the very bad.
Obviously no company on earth has a perfect record so I'd be really interested in hearing the details of your situation so I can decide whether this is a company I should avoid using or whether you, and others, have just had an unfortunate experience.
How close to the trip are you? Had you booked a non-transferable deal price? Who did you speak to to try and resolve the issue? Were they rude and unhelpful or was it just simply in their terms and conditions that you couldn't change the date?
And, oh, welcome to the TripAdvisor Forums.