There is much to be said about the Blender versions and which to use, to avoid being repetitive, I’ll link you up, it’s been discussed here:
Which Blender Version To Use
And a handy great article that should inform you of all your inquiry:
https://blog.cgcookie.com/top-7-tips-when-starting-to-learn-blender/
But in short: You can open any file with any Blender version, your file and your work doesn’t have to stay with the Blender version you created and worked with. That said since there are a few changes here and there between versions, if you used a new feature that is not present on older versions and you open that on a old version that lacks that feature, you’re going to get a couple of warnings. That is not a common problem though.
Yes you can upgrade to any new version at any time. Long term support is more for if you’re working on a long production like a movie that will take years, your everyday person doesn’t really need to worry about the long term supports.
