Migrating entities with unknown fields?

We are currently trying to create our first migration setup. We're migrating a certain node, which has a number of fields (title, description, etc, some of which are Entity references).

As far as our current understanding of this (great!) module goes, we should hardcode these fields in our migration class code. So if an admin adds/removes a field for this content type, the migration will not work correctly.

Is this correct? If not, what direction should we look to make our migration script more dynamic?

Thanks!