While I would not hesitate to take a luxury bus from KUL to SIN, there is no way I would do the same from SIN all the way to PEN. Why spend more than 10hrs on the road when its only an hour plus flight.

While cheaper air tickets do have restrictions attached, I am sure most of the luxury bus tickets would have similar restrictions as well ( I believe some may allow you to change dates up to something like 3 days before departure). Your dates are only a few days from today, I am not sure how the luxury bus is giving you more flexibility.