re: So, are you suggesting the copied field as a test just in case something goes wrong?
When you copy a field it copies the data in all the records too.
As for preserving the line feeds, I have not ever tried to do that. I don't have time to test right now, but its possible you can make a Formula Rich text field with a formula
SearchAndReplace([my text field], "\n", "<br>").
Then if that looks visually OK, try changing THAT field from a formula Rich Text to just Rich Text. If you get a chance to test before I do let me know if that works.