No, it was not difficult. There is no real "conversion" necessary, the wheels bolt on and fit with no clearance issues. The only modification necessary is to the brakes, the bosses are are not in the right spot. There are two options for the brakes, the simplest one is to use brakes with pad adjustment slots long enough for the pads to center over the rim, or to use offset adapters.

The bike is noticeably faster with 20" wheels, and the ride is more stable, but the bike does not fold as compactly, though it setill easily fits into a Birdy bike bag.