Sorry - but there is no 'forward' compatibility between the models.

Is there a way to convert Y2U tot X8U? Or can I simply rename it?

No - and No. The new format is radically different from the old. The old models don't have any knowledge about the new format used by the M models.

When updating (down dating) I would like to load a ModxM-userfile (Y2U-newest) to a Modx+ (X8U-old) board.

Have you considered doing it the other way around? Load a Modx+ file to the M model? The M models DO HAVE knowledge about the format used on the old models and can load their files.

And you can rename an old user file to have the extension of the old library file and load it as a library file.

I have the montage, genos, and every type of modx. For ergonomic purposes at some gigs I only carry 1 or 2 of my modx's.

Why not leave the Modx at home and carry the Modx M instead. That would solve your problem.

I develop on the M, but I like to use my old keyboards for backup or as second tier.

If what you develop is intended to be used on the old model then you need to develop it on the old model. Then you can use it on both old and new.

Of course I understand that ANx is not supported in the old keyboards. But the remainder should stille be able to load. When trying to load the Y2U file on the Modx+ it doesn't appear on the screen. 

The formats are different. There are no 'converters' that we are aware of.

The older generation doesn't load the new generation content.

Soundmondo doesn't convert between M and non M.

ESP only saves Y2* files so that doesn't do it.

JM tools doesn't claim to do this but the tools for M seem like they're not complete compared to past generations.  You may want to ask Melas to consider doing something to provide a converter.

Doing sound design on the MODX (non M) is best since the files it creates are fully compatible with the M.  And ESP.  

Thanks Toby and Jason. So the best solution is to buy another compatible modxm7.

Have you considered using the ESP and taking your laptop/esp to gigs?

The ESP is basically the M on a PC. You don't need to have the M connected (or at a gig) to play the performances using the ESP.

But if you need to use the older model for a gig then you need to do the development on that older model.