Perhaps interesting to mention - I noticed Transavia (Holland) operating an A320 today. Specifically
this flight ATH-EIN with registration LZ-MDK.
The aircraft is probably a leased one (especially looking at the registration), though unclear to me if that means Transavia crews? I'm guessing it would be too early for Transavia/KLM crews to be trained on the A32x platform since deliveries are still a year away?
The owner if this aircraft (fly2sky) do seem to specialise in training crews to migrate to a new platform. Though they also do regular charters.
Simultaneously knowledge is being transferred from Fly2Sky, so that the client carrier can build its own skills and, in time, bring an outsourced solution in-house.