Normally agents will not allow this, particularly if DFW or IAH. AUS may be more lenient. If it's an overnight stay then they will allow short check. If the flight gets amended by changes from BA then they can indeed switch to LCY or LGW, or put in an overnight stop. Even better to find the same fare bucket and actually use that ticket at some distant point in time. If the agent doesn't budge on the issue then you have to ask for it in baggage reclaim, and allow a few hours before it is retrieved to the carousels.