In the US it is fairly common to make quick transition courses. In the 80s I went from the vertical twin jet to the F15 on exchange, a short transition course was followed immediately by an instructor pilot course, before an instructional tour. I would not say it was easy, but it obviously can be done, I believe all RAF exchange officers made the successful transition to F15 IP.
As an aside Ken Hare visited while I was on my exchange tour and was shocked to find me as a QWI (not QFI) teaching ab-initios to fly the Eagle.