How to optimize UPDATE with a nested SELECT subquery?

I wrote a complicated UPDATE query, and it works, but it looks menacing. Here's what I'm trying to do: In each topic user 'Bob123' posted anonymously. When you post anonymously in a topic, you get a unique anonymous index for that topic. Say I want to merge two topics together. Bob123 has a different anon index in both topics, so his unique anon index wouldn't be unique. I only have two pieces of data to work with: $topic_id, the topic id you are merging into, and $post_id_list, all the post ids that got merged over. I want to update all anonymous_index entries per each distinct poster_id's post in that topic. This anonymous_index needs to be the original index they had in the topic before the other topic was merged into it. The first SELECT query first selects the anon indices of the moved posts. The outer SELECT query gets the first non-merged post's anon index (if it is > 0) of those merged posters in the topic and selects a merged anon index from the first query. Then, I update it. Wherever the anon index of those posters in that topic doesn't equal the old index, I update it. Is there something simple that I'm missing here? I don't like the fact that I have a subquery in a subquery. At first I was using HAVING MIN(anonymous_index) MAX(anonymous_index) along with AND post_id NOT IN ($merged_post_list)to select the poster id list that needed to be updated and an unmerged anon index, but it returned 0 rows with this. If the merged post is BEFORE all original posts (and has a larger anon index), then the minimum anon index will match the maximum index for that poster. So making another subquery fixed this...

$merged_post_list = implode(',', $post_id_list);

...

UPDATE " . POSTS_TABLE . " AS p
INNER JOIN (    SELECT p.post_id, p.anonymous_index AS old_index,
                       merged.poster_id, merged.anonymous_index AS new_index
                FROM " . POSTS_TABLE . " AS p,
                (       SELECT poster_id, anonymous_index
                        FROM " . POSTS_TABLE . "
                        WHERE post_id IN ($merged_post_list)
                        AND topic_id = $topic_id
                        AND anonymous_index > 0
                ) AS merged
                WHERE p.post_id NOT IN ($merged_post_list)
                AND p.topic_id = $topic_id
                AND p.anonymous_index > 0
                AND p.poster_id = merged.poster_id
                GROUP BY merged.poster_id
) AS postdata
SET p.anonymous_index = postdata.old_index
WHERE p.topic_id = $topic_id
AND anonymous_index > 0
AND anonymous_index  postdata.old_index
AND p.poster_id = postdata.poster_id

post_id is the primary index, poster_id and topic_id are also indices. Here's some sample behavior: Before merge:

|post_id_____poster_id_____anonymous_index|
| 11         | 3           | 2            |
| 12         | 22          | 1            |
| 14         | 22          | 1            |
| 15         | 3           | 2            |

After merge:

|post_id_____poster_id_____anonymous_index|
| 10         | 22          | 4            |
| 11         | 3           | 2            |
| 12         | 22          | 1            |
| 13         | 3           | 4            |
| 14         | 22          | 1            |
| 15         | 3           | 2            |
| 16         | 22          | 4            |

After UPDATE (the above query):

|post_id_____poster_id_____anonymous_index|
| 10         | 22          | 1            |
| 11         | 3           | 2            |
| 12         | 22          | 1            |
| 13         | 3           | 2            |
| 14         | 22          | 1            |
| 15         | 3           | 2            |
| 16         | 22          | 1            |

EDIT: I've made the following index and an alternative SELECT query to avoid having two subqueries, how would these fare?: (topic_id, poster_id, anonymous_index, post_id)

SELECT merged.poster_id, merged.anonymous_index AS new_index,
	   old.post_id, old.anonymous_index AS old_index
FROM " . POSTS_TABLE . " AS merged,
	 " . POSTS_TABLE . " AS old
WHERE merged.post_id IN ($post_list)
AND merged.anonymous_index > 0
AND merged.anonymous_index  old.anonymous_index
AND old.topic_id = $topic_id
AND old.post_id NOT IN ($post_list)
AND old.anonymous_index > 0
AND old.poster_id = merged.poster_id
GROUP BY merged.poster_id
ORDER BY NULL

EDIT AGIAN: Here is my schema:

Table structure for table phpbb_posts
--

CREATE TABLE phpbb_posts (
  post_id int(10) UNSIGNED NOT NULL,
  topic_id int(10) UNSIGNED NOT NULL DEFAULT '0',
  forum_id mediumint(8) UNSIGNED NOT NULL DEFAULT '0',
  poster_id int(10) UNSIGNED NOT NULL DEFAULT '0',
  icon_id mediumint(8) UNSIGNED NOT NULL DEFAULT '0',
  poster_ip varchar(40) COLLATE utf8_bin NOT NULL DEFAULT '',
  post_time int(11) UNSIGNED NOT NULL DEFAULT '0',
  post_reported tinyint(1) UNSIGNED NOT NULL DEFAULT '0',
  enable_bbcode tinyint(1) UNSIGNED NOT NULL DEFAULT '1',
  enable_smilies tinyint(1) UNSIGNED NOT NULL DEFAULT '1',
  enable_magic_url tinyint(1) UNSIGNED NOT NULL DEFAULT '1',
  enable_sig tinyint(1) UNSIGNED NOT NULL DEFAULT '1',
  post_username varchar(255) COLLATE utf8_bin NOT NULL DEFAULT '',
  post_subject varchar(255) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL DEFAULT '',
  post_text mediumtext COLLATE utf8_bin NOT NULL,
  post_checksum varchar(32) COLLATE utf8_bin NOT NULL DEFAULT '',
  post_attachment tinyint(1) UNSIGNED NOT NULL DEFAULT '0',
  bbcode_bitfield varchar(255) COLLATE utf8_bin NOT NULL DEFAULT '',
  bbcode_uid varchar(8) COLLATE utf8_bin NOT NULL DEFAULT '',
  post_postcount tinyint(1) UNSIGNED NOT NULL DEFAULT '1',
  post_edit_time int(11) UNSIGNED NOT NULL DEFAULT '0',
  post_edit_reason varchar(255) COLLATE utf8_bin NOT NULL DEFAULT '',
  post_edit_user int(10) UNSIGNED NOT NULL DEFAULT '0',
  post_edit_count smallint(4) UNSIGNED NOT NULL DEFAULT '0',
  post_edit_locked tinyint(1) UNSIGNED NOT NULL DEFAULT '0',
  post_visibility tinyint(3) NOT NULL DEFAULT '0',
  post_delete_time int(11) UNSIGNED NOT NULL DEFAULT '0',
  post_delete_reason varchar(255) COLLATE utf8_bin NOT NULL DEFAULT '',
  post_delete_user int(10) UNSIGNED NOT NULL DEFAULT '0',
  sfs_reported tinyint(1) UNSIGNED NOT NULL DEFAULT '0',
  parent_id int(10) UNSIGNED DEFAULT '0',
  post_depth int(3) UNSIGNED NOT NULL DEFAULT '0',
  is_anonymous tinyint(1) UNSIGNED NOT NULL DEFAULT '0',
  anonymous_index mediumint(8) UNSIGNED NOT NULL DEFAULT '0'
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_bin;

--
-- Indexes for dumped tables
--

--
-- Indexes for table phpbb_posts
--
ALTER TABLE phpbb_posts
  ADD PRIMARY KEY (post_id),
  ADD KEY forum_id (forum_id),
  ADD KEY topic_id (topic_id),
  ADD KEY poster_ip (poster_ip),
  ADD KEY poster_id (poster_id),
  ADD KEY post_username (post_username),
  ADD KEY tid_post_time (topic_id,post_time),
  ADD KEY post_visibility (post_visibility),
  ADD KEY parent_id (parent_id);

--
-- AUTO_INCREMENT for dumped tables
--

--
-- AUTO_INCREMENT for table phpbb_posts
--
ALTER TABLE phpbb_posts
  MODIFY post_id int(10) UNSIGNED NOT NULL AUTO_INCREMENT;COMMIT;