Do I really need to replace the whole plastic pan (eg 1087.298.437), or can I just change the oil?

On most modern 6 and 8-speed ZF transmissions, the filter is permanently built into the plastic oil pan. You cannot replace the filter separately. Additionally, these pans are prone to slight warping over time; installing a new pan ensures a fresh seal and provides a new magnet to capture any metallic wear particles, protecting your transmission's longevity.