I gather that in the event of late notice unit swaps then RTT may take some time to update, as the systems the industry uses will need to be updated manually. When I was a shunter it wasn't unusual for control to ring and ask us to let them know what units are on a particular train, or ask if we have a particular unit in the depot.
The systems will have to be updated eventually, especially to keep track of milages.
However, if a train is updated, say, 24 hours after its ended it's journey, would RTT (and for me, more importantly) Railmiles also update?