Saving CorelDRAW Files to Previous Versions May Introduce Problems
First, the two versions of the software were created six years apart. In the world of technology, that's an eternity. Let's go over two specific types of objects in CorelDRAW and how they were handled in the two versions. The differences in these two objects illustrate how things can fail when saving backwards.
A rectangle seems to be a really simple shape and it is a special flavor of object in CorelDRAW. Since the early versions, we could round the corners of a rectangle. With CorelDRAW X5 (if memory serves me correctly), two more types of corners could be specified. Even better, no longer would corners be distorted if the rectangle had been resized disproportionately. So a rectangle created in CorelDRAW X6 and saved backwards to CorelDRAW X3 may have to be converted to curves to look exactly the same. That's great, unless you want to edit it.
For most users, every file with contain some text. Often files will contain quite a few pieces of text. CorelDRAW X6 added support for OpenType and therefore a lot more characters. If you save a file to any previous version, the text will be converted to curves if the look of the text needs to be maintained. If you do that, the text isn't editable. Sure, you can leave the text editable, but the look could change.
Moving files from older versions to newer versions typically works well. Remember, the new version knows what was possible in older versions. The older versions didn't have a crystal ball to know of features that would be added in a future version.
I've only discussed two very basic types of objects. Expand this discussion to the many types of objects you can create in CorelDRAW and I hope you realize that files may not go backwards very well, especially when you save so many versions backwards. If it works well for you, it is cause for celebration. Just don't be surprised if you run into some glitches.