Thanks for the above comments.
just one further “out of interest” query, how does it work for the crews? There may be aircraft changes scheduled in advance but for others how far in advance do crews get notice? So for example if a NCL and MAN based 738 need to swap for a maintenance issue would the crews change aircraft at the outstation so that they end up back at their base but on a different aircraft or could the crew on the aircraft just be told they are going back to MAN rather than NCL. Always wondered!