Answer

Your only chance will be to export all your already processed files as 16bit-TIFF (lossless and preserves most of the quality) and import them to your new software.

XMP just standatizises the way "how" metadata is stored in a dng or a sidecar file. But it says nothing about how this metadata should be processed by the application.

Example

You use in Lightroom the "clarity"-option and set it to 42. Now in the sidecar file something you see something like this (extremely simplified):

<clarity>42</clarity>

Now if you take that to another application it may ignore the clarity "tag" or do something entirely different, depending on the algorithm this software is using. And of course, the algorithms of the Lightroom-Development Module are proprietary to Adobe.

If you export to a tiff, all those tags will be applied a copy of your real image-data (thus: changing pixels) and a new file is saved. You can watch this file in any software you want, without knowing how to apply clarity because that has already happened. But as you can see, you can't go backwards anymore and change the clarity value, so your edit-history will be lost if taken to the new application.

For more about xmp and dng, please refer to my answer over here.

This does not only apply to Lightroom, but to any software which uses a non-destructive approach.