Yes, it is possible to replace a uPVC door with a composite door. However, it is important to ensure that the new composite door is compatible with the existing frame and that it fits properly. In some cases, the door frame may need to be replaced or modified to accommodate the new composite door. This will depend on the condition of the existing frame, its size, and the design of the new door. It is recommended to hire a professional installer to replace the uPVC door with a composite door. A professional installer will ensure that the door is properly installed, sealed, and insulated to prevent air leaks, water infiltration, and energy loss.