I expect that it has much more to do with borders staying open, and whatever effect their flexible status has had on the willingness of people to book. If the bookings aren’t there, it takes very little effort to substitute one type for the other, especially as they aren’t doing anything else at the moment.
BTW, there is no such thing as a proving flight in this instance. Test flights could well be necessary for some aircraft as they come out of storage, but they don’t involve a wander across the country. There are very specific items to be tested, with many of the switches that the pilots never get to play with, being exercised. For instance, on some flights, engines are shut down and then restarted in flight.