change fields in content type after creation
Have you tried to peek inside a database via PhpMyAdmin ? For every field you have field_data_field and field_revision_field table, containing several columns depending on the field type. And also some config tables like field_config and field_config_instance, so many things to be aware of.

This is more an advice than a clear answer, meddling with database may be risky task.

But I do understand if you have large data collections, you might try to risk to hack database to convert to other field type.

Beware that field tables may be referenced by other tables / modules, so backup your db first and test on clone.