Kyma 7 is backwards compatible, meaning that any Sounds or Timelines that you created in Kyma X can be opened in Kyma 7.
However, as is the case with any new software, files created in the new software may not be openable in the older version (since there are new features that did not exist in that older version). In other words, Sounds and other files that you create and save in the new Kyma 7 may no longer be openable in Kyma X.
Therefore, before even launching Kyma 7, we recommend that you first make copies of all your Kyma files (.kym, .ktl, etc) and put them into a folder called Kyma X Archived Files.
That way, you can continue working on your current projects knowing you have a backup (though, once you've tried Kyma 7, it is highly unlikely that you'll ever want to revert Kyma X).