>Is there any way to convert menu.mpr back to menu.mnx and menu.mnt ?

At least, there is no easy, built-in way. Nor do I know of any third-party utility that will do this for you.

You are supposed to do all your changes in menu.mnx + menu.mnt, and then generate the file menu.mpr again.

If you need to do manual changes to the MPR (this may be necessary for language customization, since in some places you can use constants, but not variables, in the MNX), just use your MNX as the original design, and then make any changes manually to the MPR. In this case, forget about the MNX, and just use it as a quick guide, if you don't know how to do something (transferring changes manually, from one MPR to another). But in general, it is better to do all your changes in the MNX, and generate the MPR.