WebPlus X2, WebPlus X4 and above
WebPlus X2 is the first version of WebPlus to use the new XML file format and so all versions from X2 onwards will be able to open files made in later versions.
You can open an X4 file in X2, but all new objects and features introduced in X4 will be stripped from the file. As such, it is recommended that you save the file to a new file name using X2 so that the original version does not lose its X4 features permanently.
This will be true of all future releases (X5 and onwards) which also use the XML file format.
All versions of WebPlus continue to be backwards compatible, so newer versions can still load files from older versions.