Good question. When I replaced my engine oil cooler, I had all new parts ready to go. New oil cooler, and 3 new coolant pipes. The new engine oil cooler comes with all the attaching seals. And the new pipes come with their all seals. Even though I might have been able to reuse the old cooler and dissemble it, and replace the internal seals, I didn't bother. Because i wanted to get the repair done as quick as possible with the least possibility of complications. If you have lots of time, it might be worth the bother to dissemble the old engine oil cooler and rebuild it with new internal seals. If you are careful, you can probably reuse the 3 coolant pipes if you install new seals on the old coolant pipes.
Deano