A number of smallish changes, applied across multiple large codebases? Yeah, that's a lot of work. Our software won't have to be completely scrapped and started over, but we will have to spend quite a lot of time porting.

In other words, if you have five hundred instances of `print "foo"` which all need to change to `print("foo")`, it's not that you have to "completely rewrite", but you _do_ have five hundred changes to make. And since applying a change is almost never as simple as a single search/replace/rename, it means work.