This is like clone-indirect-buffer but it only clones the text -- not the text properties.
Put jac.el somewhere in your load-path. Maybe compile it.
Add the following line to your init file:
(autoload 'jac "jac")
Call the command jac for cloning the current buffer.
Both buffers have the same text contents. If you edit the text in the original buffer the modifications are transferred to the clone and visa-versa. Text properties are not transferred. That means you can switch the major mode for the clone.