Dear Maha, it is a privilege that you wanted to opt for and then you decide that you don't want to. So, its absolutely ok even if you have decided to opt for Customary dressing but then decide that you are fine with the NTA dress code. The customary dressing is in place to respect Individual's religious beliefs and not to offend them. Say, for example, I opted for Customary dressing thinking maybe I want to wear a Sweater or something like that thinking it would be cold on the day of the examination hall. However, on that particular day, it is quite hot and I decide not to wear any dress that falls under customary dressing. I follow the general dress code and wear Trousers and attend the exam hall. I will not be held responsible for anything because I was given a choice to opt for Customary dressing and I decided not to use it. These are things that can be and will be ignored. So, don't panic and focus on preparing for the exam. Good luck!