I traded a eos 5 mark 1 for a used 1DS III a couple of years ago. I love the 1DS, but it heavy, so I just purcahsed a used 5 Mark 1 at a very good price. The picture quality almost meets the 1DS. It focuses a little slower and is missing a coulple of features I like, but it still takes great pictures.

You will have to replace your lenses only if the lenses to have are designed solely for APS cameras.