Change CCK field data type
Based on the comments on your question, I certainly hope you're doing this in a .install file and not a .module file.

The $ret variable is expected to be a reference, so you'll need to declare it before hand. 

$ret = array();
db_change_field($ret ...);


Is your table name really content_type_?

And you may want to look at this and stick in a couple dsm() calls to see what the db_change_field() function is actually passing to MySQL and then you could trouble shoot the query directly.