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When you are implementing hook_schema_alter(), the table's schema is not changed.
You are informing the modules calling drupal_get_schema() (which is then called by functions such as drupal_write_record()) what the actual schema is. If you altered a database table without implementing hook_schema_alter(), the functions/modules using drupal_get_schema() would get the schema implemented by the module using that database table, which is not anymore the one describing the database table.

You need to first alter the existing database table using db_change_field(), taking in consideration the following note:


  IMPORTANT NOTE: To maintain database portability, you have to explicitly recreate all indices and primary keys that are using the changed field.