I cannot speak for SAS but to give you an understanding of the cross crew qualification (CCQ) course used to upgrade experienced Airbus pilots amongst the A318/319/320/321/330/340 aircaft.
The A318/319/320/321 (A320 base) is the one type rating, the 330 (A330 base) and 340 (A340 base) are separate type ratings. To convert from a 321 to 340 would require a CCQ course of about two weeks. This is dependant on the local regulator.
The times required for the courses to go from one base to another is as follows :
A320 → A330: 11 days CCQ course
A320 → A340: 13 days CCQ course
A340 → A320: 11 days CCQ course
A330 → A340: 3 days CCQ course
A340 → A330: 1 days CCQ course
This is assuming the pilots has experience on the “base” aircraft, practice on “new” aircraft, undertake refresher training on both types, proficiency checks on both types.
A number of operators do undertake mixed fleet flying.
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