Engine interchangeability
As far back as the A300-600 and A310, the pylon design enabled either the GE or Pratt engines to be fitted. The different suspension arrangement of the RR RB211 meant that it would have had to have a specific pylon design, which was one reason for Saudia choosing not to have RR engines.
The later versions of the RB211 for the A330 and A380 have re-designed mounting attachments and allow interchangeability, though I assume that this is at contract negotiation stage, rather than in the field - I doubt that A380 operators would be able to do a kind of "mix & match" within their fleets, let alone on the same airframe. *
* This is not to suggest that any Ppruners have even remotely suggested this, and is not meant as a criticism of those who mentioned "interchangeability", rather as a kind of reductio ad absurdum.