Is JPA provider portability (swapping Hibernate for EclipseLink) actually easy in practice?

In principle yes, since application code targets JPA's standard interfaces — but in practice, most non-trivial applications end up relying on at least some provider-specific features (Hibernate-specific annotations, query hints, caching configuration) that fall outside JPA's standard contract, making a real provider switch require reviewing and rewriting those specific pieces, not a true drop-in replacement.