Boggles the mind and makes no sense????
You have been using computers for more than a year or two right?
Just saying - this is just common sense (as in everyone knows this) and is pretty much standard on 95% of software for the past 20 years. Newer versions of software introduce newer features, etc. that an older version will not understand or be able to deal with. Its pretty much a common standard in just about all software and programs that a newer version can generally open files created in an older version but an older version more often than not cannot open a file created in a newer version. This is why many programs give you the ability to export or save to a lower version. Flexi - its under export, Illustrator (or Photoshop) - its under the save options that come up when you save, Office apps (Word, Excel, etc) its under Save As. If this was not common all these programs would not bother with giving that capability.
Sometimes it might work in some programs though. For example, a file originally made in Flexi 8.6, then subsequently used in Flexi 10, will probably still open fine again in 8.6 again even after saved over in version 10. Unless some specific feature that is new to version 10 was done to the file - then 8.6 might freak out if you tried to open it there again.
Not trying to be harsh or anything. I honestly though this was just common sense that files from newer versions of programs (just about any programs except maybe a txt file) very likely will have problems or not open at all in an older version of the same program.