I did it the other way round, went helicopter to big fixed wing. Spent 8 years (the lost years) on Boeings and Airbus then returned to helicopters.
Should have done only 4 years but it was good money for old rope money so I carried on long after terminal boredom had kicked in.
The conversion exams are not a problem, where the problem lies is the cultural differences. I did the change late in my career and I think it would have been easier if I had been younger. I always felt behind the pace in comparison with the guys who had trained and developed within the big fixed wing world.