Found it. It was under the BE T&Cs:
- Changing a flight: Ticket changes are not allowed. This includes advance changes and same-day changes, even for Premier members. Basic Economy tickets can only be changed if they’re issued between March 3, 2020, and April 30, 2021; or between May 1, 2021 and December 31, 2021, for travel commencing between August 11 and December 31, 2021.