[QuotedForTruth]

They will stay for 1 to 4 months. Its apparently partially dependent on whether they are prepared to launch the first actual crew mission on Aug 30.

A crew swap would require astronauts that are on station to return on the dragon, something they may not have trained for. So to do a swap, you'd probably have to take more than just the two test pilots up since they are likely required to pilot the capsule back down. Taking additional crew on the test flight would certainly be more risky to those additional lives.