A: 

Try: update wp_posts set post_content = concat(post_content,'\n');

I tested with a mediumtext field here and worked..

Torment
A: 

I don't know where my brain was. I was using HeidiSQL as the interface to MySQL, and wasn't seeing the change in the database. But it did change, which I saw once I refreshed the table data. Sorry for the false question!

Daniel Jacobs
you might be able to delete the question
Yar