Firstly, a parts list is not a specification. In fact, it is reasonable to develop your specifications even before you select part number 1.
Despite that, this is a semantic argument that you're embarking on with your instructor, and certainly doesn't bode well for you. If when you wrote your document, you actually meant solenoid and thought that "DC motor" included solenoids, then you didn't communicate effectively. Effective communication is one of your most valuable engineering skills, and it sounds like this exercise is teaching you this. If I had an engineer working for me that sent a semantic argument like this on my desk, instead of explaining to me how this change would help me solve my problem better, I'd have some issues with his level of professionalism.
Your response should be "Ongoing research showed us that a solenoid will enable us to meet our specifications better than a DC motor", then make your case, show how it doesn't adversely impact your bottom line or your Gantt Chart, show how it will enable you to meet your actual specifications, and ask to use the solenoid. If you can meet your specs with a DC motor, and the change to a solenoid is at all inconvenient, the DC motor may well be the way to go. (As an aside, an engineering team that consistently exceeds specs instead of just meeting them might well be wasting time and money). Maybe your instructor is actually looking at a slightly bigger picture than you are, and for some reason thinks that the DC motor is a better option.
In any case, as one who teaches design, I suggest that you'll have a much better experience if you communicate with your instructor instead of arguing with him.
Note that in our class, we get actual customers with actual problems for our teams instead of just letting students select their own project, mainly to enhance customer interviewing, problem identification, and communication skills. Also, we try to pin down specifications and deliverables, but we realize that in a time compressed 8-month experience, expectations and achievements can differ and flexibility is required. Changes are carefully tracked, though. So far as a change like the one in this question goes, however, identifying an approach through research that results in a positive change before being absolutely committed to an expensive and time consuming option is to be applauded, not discouraged. Of course, we're seeing half a picture here, and this instructors approach might be optimal for this situation.