Can you really not change field names in the Legend?

You can change the title of the Legend itself, but as for the field names I don’t see a way to change them.

As you correctly say, you cannot change the content/values of a field in the visual itself. In other words, the legend content.
As a workaround, you can use an IFELSE calculated function to rename each value within the field as you wish and use the newly calculated field in your chart.