Why is this mySQL UPDATE not making changes?

I know nothing about mySQL. A friend and I are struggling with this tiny bit of code.

Background:

I'm trying to solve an issue I have with character encoding on my WordPress database. My original WordPress database is wordpress, and the modified database is wordpress2 (which exists largely as a copy of the original). I want to select the post_content of a single entry in wordpress, convert it to utf8, and insert it into the corresponding row in wordpress2.

The Script:

update wordpress2.wp_posts wp2 
   set wp2.post_content = (select convert(wp.post_content using utf8)
                             from wordpress.wp_posts wp
                            where wp.ID = wp2.ID )
 where wp2.ID = 789;

When run:

Query OK, 0 rows affected (0.00 sec)
Rows matched: 1  Changed: 0  Warnings: 0

Why isn't this changing any rows? What do we need to do to make it do so?

Thank you.