This question can only be answered by the architect or architectural technologist who prepared the plans on your behalf. Fundamental issues come into play, such as the availability of load-bearing internal walls right down to foundation level or steel beams in certain locations where a wall is not available. If your drawings show a standard trussed rafter system it is likely that no allowance has been made for a traditional cut roof alternative. You could consider attic trusses which, depending on the length of span, may be able to span the upper floor without internal support – although this method would not give you a vaulted “star-gaze” ceiling to the first floor. Given that the property will be at plate within the next few weeks, I suspect that you will be unable to resolve this now without delaying the progress of your build.