I bought Scrivener 3 from the web site since I am informed my current version bought from the App store will no longer function with later versions of OS.
I saw some help files on this web site, describing how I link to my existing Scrivener files in this scenario, since I may have to change document location information. For the life of me I cannot find those instructions again now I have bought the product. I use a dropbox location for the files
In general I would say that would not be necessary, but I don’t really know what you mean by “linking” to files. The App Store version is sandboxed, this is true, but sandboxing is more of a technical limitation than something that changes how you work with projects. If you’re having troubles locating where your old projects are, have a look at the File ▸ Find All Projects in Spotlight menu command. If it isn’t clear, Scrivener opens projects off of the disk like just about any program would. While a project is way more complicated, it’s functionally no different than loading a JPG in Preview or something along those lines. You just have to find the JPG and load it. No need to move it around just because you switched to a different program.
As for the instructions you saw, you probably came across this one. That may have some useful bits of information for you, especially if you’ve created a number of presets and templates in the past—but as it says at the top, it’s an optional procedure.