I'm not sure of the details surrounding your specific airlines, but I can speak for BWIA of Trinidad and Tobago..They've undergone a fleet renewal program and I knew a pilot who was reassigned from the Dash 8 to the 737-700 (that's when they had 9Y
-TJI back in 1999)..I thought that was a huge switch but I understand that it is possible due to the retraining programs that many airlines have..for those like BW
without flight simulators, we get the pleasure of watching the airplanes do repeated touch and goes, overshoots etc...as the pilots build up their number of contact hours with the new type of aircraft..
On a different note, some airlines want fleet commanality so that they won't have to hire as many pilots as they could make the transition without the extra training and money it'd require..If I'm not mistaken, I think the MD
-90 had the "problem" that some of its features like cockpit layout(?) was different from the -80 series so airlines would have a slight problem should they have a mixed fleet....
so all in all, I guess fleet commonality is really important these days especially when so many airlines find it necessary to cut costs in all arenas in order to return to the black..so one way in which they're doing this is to standardize on a few aircraft models...