There have been times, especially in art school when I've been tempted to paint over an old painting and the results have almost always been regrettable. The impasto lines or passages of the old painting always show up in the new painting where it's not wanted like the texture from a tree in the old paint ruining the planes of a face. You could sand or scrape the texture off the old painting but that creates a slickness on the surface that the new layers of paint won't adhere to. Over time the old images will show through the new, which could look kind of interesting, but you never know. Paint remover isn't an option either, it contains a wax that will stick to the canvas and isn't good to paint on.
The only solution is a new canvas or board and take a razor blade to the old canvases. At least the stretcher strips are reusable.