You should refer to the title deeds or deed of conditions for your property to ascertain your position on changing the appearance and materials of your windows. In addition, you should contact your local authority to ensure that your proposal meets current planning regulations.
If your title deeds do not stipulate that there is a requirement to maintain uniformity within your building or development, you may be at liberty to change your windows. In this case, your windows are a private matter to you and you do not need to notify the factor for your development. You should still ensure that your proposed change meets planning regulations.