Hi, just because there doesn't seem to be any clear-cut information on the subject I'd say you might want to go at it in stages.

And of course, take database backups at every stage.

Things that typically break between the 5.7 branch and the v8 branch are custom attributes although not always as the core team put a lot of effort in trying to make them backward compatible. But that was between 5.7 and the first few v8 versions. What happens after that I can't say for sure. I know some custom attribute packages in the marketplace install fine on both systems but then refuse to uninstall on v8. Little problems like that.

There were a few minor changes in how certain function works. Not the widely used ones but still they can potentially break a package.

These days Concrete5's slack channel is a good place for technical questions. You might want to give it a gohttp://www.concrete5.org/slack