When can I choose or change a British Airways seat without paying?

What this answer depends on

• British Airways published advance-seat-selection rules, including Basic, First, status and partner-marketing conditions
• For a fare that permits free choice at check-in, waiting changes when you may select without paying; it does not promise that a preferred location will remain. Basic is different: BA describes automatic allocation at check-in with payment to change the allocation. Read the actual fare instead of applying the general free-at-check-in statement to every Economy ticket.
• BA identifies free selection from booking in First. Its Bronze benefit generally begins seven days before departure, while long-haul exit-row choice has a later check-in qualification. Higher tiers have their own rules. These are entitlement checks, not reasons to invent a complimentary upgrade or to assume that a companion on another reservation qualifies.
• BA says certain package or cruise bookings cannot buy seats until the ticket has been issued. A reservation visible in Manage My Booking can therefore still be awaiting the condition necessary for paid choice. Ask the provider to confirm ticket issuance rather than treating a missing payment button as proof that every seat is occupied.
• BA’s conditions restrict changing a paid seat on a BA-operated flight marketed by another airline. For eligible BA-marketed/operated changes, a higher-priced replacement can require a difference in price. That channel distinction is why the answer should not be reduced to “all BA seats can be changed online.”
• BA generally permits free seat choice at check-in 24 hours before departure; Basic receives an allocation with a charge to change.
• BA First has free selection from booking; Bronze generally receives free selection seven days before departure with a later long-haul exit-row qualification.
• Some BA package/cruise reservations cannot purchase seats until the ticket is issued.
• BA restricts changing paid seats on BA-operated/other-airline-marketed flights; eligible BA-marketed/operated higher-priced changes can require a price difference.