I once had a change of plans but only could change the date of the outbound to the same destination, and not to the destination I needed to go. Inbound did not need a change. A change fee (forgot how much but 150 Euro or thereabout) plus price difference applied. I ended up booking a one way from my original destination to the new one taking a one night layover as precaution. So I think no chance you can change your ticket in the way you want (starting in OTP instead of Sofia and going to PVG instead of Thailand). It is likely the system prices in a cancellation fee of the original ticket maybe?