A "graphical" Programming system such as LabVIEW probably has version-specific "stuff" embedded in the "code" (the .vi, the .ctl, and other files). With Matlab, or C++, or Python, the "program" is simply text, and going from Version 2017 to Version 2020 doesn't change anything about the underlying text.
But with LabVIEW, the .vi file has stuff embedded inside it that was created by the version of LabVIEW (say, LabVIEW 2018) that created it. NI, to their credit, has arranged that later versions of LabVIEW (say, LabVIEW 2020) can look at the VI, see its Version number, and while opening and reading the code, store the information internally in its current format. As long as you do not save the VI, overwriting the older representation, you can examine the code, even modify it (but don't save it with the same name!), even run it (but don't let it be saved!).
Oops -- you accidentally saved it! Now you cannot open it with the older version (it "complains" that you are opening this VI, saved in LabVIEW 2020, with LabVIEW 2018, for example). NI has you covered -- as Altenbach explained, you can go to the Version that can open it, open it in that Version, then go to the Save menu and choose "Save for Previous Version". It will create a folder for all of the VIs it creates in the version you specify -- you might find lots of VIs in this folder, VIs that you already have in the older LabVIEW installation on your disk -- you can ignore these.
Hope this clarifies the situation. Dealing with multiple LabVIEW versions can be a bit tricky, which is an argument when learning LabVIEW to stick with a single (preferably recent) Version and hold off upgrading for at least a year to give yourself plenty of time to gather expertise in using LabVIEW.
Bob Schor
