The first time i heard of this a few years ago was a Britannia Nigel who came in for a PMI and was changed to LCA. He said (alledgedly) I cannot do it as i'm not prepared for the longer duty. Then I heard about a Moanarch Captain who came in for a SKG but the aircraft was tech so he was asked to wait around and ferry it to LTN, answer as above.....in reverse maybe
The CAA view is clear - you must be prepared for a Max FDP and potential discretion.
Any other offerings out there, i'm bored and need a larf