When you edit and save a Sound in Kyma 7, there is a high likelihood that it will no longer open in Kyma X. Even a Save as…, although it saves the Sound with a new name into the Sound file window, it also saves the entire Sound file to disk (including other Sounds that may be in that file). This is also the case in the Sound Browser, when you close a Sound file and it asks whether you'd like to save the changes.
That's why we recommend that, before starting your work in Kyma 7, you make copies of all your Kyma files (.kym, .ktl, etc) and put them in a folder called Kyma X Archived Files. That way, you can continue working on your current projects in the new software, secure in the knowledge that you have a backup of your work as of today's date.
There shouldn't be any need to switch back and forth between Kyma X and Kyma 7; just go for it!