Hi
Currently an "Upgrade" or "Latest" version of VP is made available in the light of users finding problems or bugs in the current version which, in some situations might be a show stopper. IMO these "Latest" versions perhaps ought to be more correctly called a "debugged" version.
A true "upgrade" would be a version that works just as before (i.e. without major bugs) but has some enhanced features for example that follow suggestions made byusers or in which parts of the program have been rewritten to enhance the way it works. As Borate rightly points out a "debugged" version (to use that term) is available as soon as the problem has been fixed, and if you bought your current program less than 6 months before, it can be downloaded free of charge. It may be that the you never experienced the bug but nevertheless you can still download it. In fact this scenario also applies to "Latest" versions which have enhancements (like user suggestions).....That would still be free.
However it is perfectly logical that one would pay for "Debugged" or the "Latest" version after 6 months of use.
Although you may be given the option to upgrade, (no harm in asking) there is no compunction for you to take up that offer if you find the "old" version you are using OK. At the same time trial versions (Beta versions) are available for users to test and report on or simply to try out.
Whilst it is true that later versions do not always work with older project files (e.g. vpj files) This is not usually the case except where the vpj files were created with quite old versions. It should be pointed out that some users who used VP 4.xx and 5.xx to create projects (which are pretty old) unrealistically still expect to open these vpj in much later versions of VP.
When NCH develop versions (or correct versions) with important changes the differences are listed here...
https://www.nchsoftware.com/videopad/versions.html
Limited as these change descriptions seem to be they are similar to those seen with other software.
The advice I think would be to keep with the version you are currently using if it does what you want. Update to the latest before your 6 months is up. Give it a try and see if it is better (or just the same). Revert to your old version if you feel that is better. All that is free. If, on the other hand if you have had it for more than 6 months,there is no compunction to upgrade. You can run the trial version free for a time (usually with restricted export) and then upgrade and pay if you like it. If you don't you can revert to the old version again.
Nat