Switch from Points to e-cert Possible?
 

Can I book a room for points and then later change to use a free night e-cert that is earned later and have my points returned to my MR account? When I tried to book with points, I got a message that an e-cert would be sent to the hotel, but I could not tell when that would occur and if it might complicate matters.

Yes this is possible. Just call the Marriott Rewards line when you get your e-cert and have them make the switch.

You can also call and book a rewards reservation and tell them that you will have a certificate later. Then when the certificate is in your account, call back and have them link the res to the cert.