just made another booking for a 2020 stay and quizzed the Citi concierge agent a bit on this.
1. all bookings made by Citi concierge prior to Sep 1 2019 will be rebated, without any number of stay limits for both 2019 and 2020 stays
2. any change to one of these reservations post Sep 1 2019 would vitiate the rebate
3. Citi concierge would remain the servicing travel agent for these reservations until all travel is completed