Not sure if this has been posted/discussed here, but when I started getting the POT/EXPO offers, I didn't enroll because I wasn't sure about impacts to credit reporting.
Answers/details on that and more, in writing from Amex reps, are available in this Amex community thread (should be readable even without an account):
https://community.americanexpress.com/thread/6451
In response to the credit reporting questions:
1. The Card will report as a Charge Card. The Pay Over Time feature is an internal feature & does not get reported.
2. The Card will not change to Credit Card, and will report as a Charge Card.
3. The Charge Card will report irrespective of the kind of payments being made (whether in full or per the Pay Over Time feature).
There's also a good breakout further down of the three different "Pay Over Time" options - Sign & Travel vs. Extended Payment Option vs. Select & Pay Later.