Per-Field to Per-Type Storage Corrupts Serialized Columns

This issue arises out of #402860: Fatal error: CCK data field not always unserialized, where we've been experiencing some troubles with FileField's serialized "data" column. It looks like CCK is using a raw SELECT query to pull out data, and then using drupal_write_record() to reinsert data. Currently this isn't taking into account serialized columns, so every time this happens data ends up being reserialized.

To Reproduce:

1. Install Content and FileField
2. Add a new FileField (default settings, single value) to the "story" type
3. Make a new story and upload something into the FileField
4. Edit the FileField, and change it to "Unlimited" data storage
5. Edit the FileField again, and change it back to a limit of "1"

Expected: The "field_[field_name]_data" column contains a serialized array.

a:1:{s:11:"description";s:0:"";}

Actual: The "field_[field_name]_data" column contains a double-serialized string

s:32:"a:1:{s:11:"description";s:0:"";}";

Switching the number of values from Unlimited to 1 multiple times serializes the data an additional time every toggle.

a:1:{s:11:"description";s:0:"";}
s:32:"a:1:{s:11:"description";s:0:"";}";
s:40:"s:32:"a:1:{s:11:"description";s:0:"";}";";
s:48:"s:40:"s:32:"a:1:{s:11:"description";s:0:"";}";";";

Attached is a patch which unserializes data before the drupal_write_record() to prevent this from happening. I wrote an upgrade path for FileField in the other issue (http://drupal.org/node/402860#comment-1370000), but it didn't take into account multiple serializations. Should we update it an include it as a CCK update?