I cant believe this hasn't been asked already..

There are numerous free assets available everywhere on the web…

loading them into the vault, and going to use them however can be a pain in the ***
due to “not compatible with 4.15.0” thanks to the author giving up or being lazy…

my question is, on assets that you can manually load and find out work on 4.15.0, is there a way to edit them
so that UE4 recognises them as compatible? I’d imagine it wouldn’t be to hard…

You can manually copy the assets from your local volt on your hard drive.

In Windows you can move asset packages from one project folder to another using explorer. What would be nice is a proper CMS app where one could browse the vault and migrate only a single asset from a package to the current project. The way it is now if you want just the one tree you have to migrate the entire package of 10 trees to get just the one.

Unreal will convert assets when it first opens them.
Thus, if you have assets that aren’t marked as 4.15, you can still add them to your project.
If you add them from the vault, the tool will warn you that the versions don’t match, and tell you to pick a version to use; use the latest available.
Then, when you actually open the asset, the editor will convert it, and 99 times out of 100, it works fine after conversion (because otherwise regular game content you build wouldn’t work after upgrading the engine!)