Retrofit installation, also known as replacement window installation, involves replacing the window within the existing frame and is ideal when the frame is still in good condition. It's a less invasive method, often more cost-effective and quicker than full-frame installation. Full-frame installation, often referred to as new construction windows, requires the removal of the entire window, including the frame and trim. This method is necessary for extensive renovations, if there's significant frame damage, or when changing the window style or size, though it is more labor-intensive and usually more expensive.