You should give a look at the upgrade notes, but as Nuxeo API is stable over multiple versions and deprecated some time before being effectively removed, your Nuxeo plugins will probably work without any change.

I have converted a number of plugins from 5.3.2 and 5.4.2 with different results. What you have to modify (or not) is highly dependent on what your plugin does, or more specifically what Nuxeo services your plugin uses and whether your plugin overrides Nuxeo-provided features, etc. If you have an older format MANIFEST file then you may have to update this as well (or you may want to anyways as I suspect at some point it will be mandatory).

Several of my more complex plugins required significant changes, but that's because I was extending/overriding some core Nuxeo features. But even those changes could be measured in hours and not in days.