Having the model & it's feature tree would be helpful. A sketch showing the changes you intend would also help.
Modifying the shape of the existing model is probably going to be harder. You could possibly use the Replace Face command, but I suspect that you are going to end up building an almost completely new body. You can use the Insert Part command to bring a new body into your part. You can then remove the parts of the old model that you don't need and merge the two bodies.