You don't have to replace the whole frame but you will need to make sure that you cut well above the bottom hinge as the frame will need to be well fastened above an under the hinge position to avoid the hinges tearing out. You may even need an extra long screw or 2 that goes through the replacement frame and into the subframe, that will depend on how well you are able to secure the replaced section.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!