Earlier quoted context omitted.
Migrating where there’s a 1-1 mapping of old API to new API is easy, such as the print to print() change. The hard part is when there’s a 1-N mapping, where a single name in the old API becomes multiple names in the new API. These are the most useful changes to make, because they can split apart unrelated concepts that had been erroneously represented by the same object, but require a human to determine which use cas…
Python2 had the 'unicode' class for characters, and the 'str' class for bytes. The whole 2 to 3 debacle was only because somebody thought that naming your string class 'unicode' doesn't sound nice enough or something.
I had clean Python 2 codebases which had been Unicode-safe for years and had good test coverage, so porting to Python 3 was trivial and mostly automated. The projects which were hard were the sloppy ones which conflated bytes and Unicode, and the poor development culture which lead to that also tended to mean they had limited testing, orphaned dependencies, monkey patching, etc. which made the migration hard. I also found a fair number of cases where that process fixed other bugs which had been ignored for years.