I traded a colony of bees (in a Langstroth box) for a top bar hive at the start of the year, and placed the Langstroth box on top of the top bar hive with the idea that they would just migrate downwards onto the top bars. To cut
a long story short, after 11 months, that didn't work.
So I made a hive that had a Langstroth shaped box at one end of a Kenyan top bar hive body, and the bees had built almost a full comb on the top bars after five days! Anyway, if anyone is in the same situation, I'd recommend this approach rather than the "chop'n'crop" technique of trying to cut the comb off the frames and hanging it on the top bars.