Converting existing content to a different locale
Currently it's not possible to edit a locale from the CP, which means you have to edit it directly in your db.

Edit locale:

UPDATE craft_locales SET locale = 'en' WHERE locale = 'en_us';


Edit elements:

Each element (entries, assets, users, etc.) which can be translated has a locale-column:

UPDATE craft_categorygroups_i18n SET locale = 'en' WHERE locale = 'en_us';
UPDATE craft_content             SET locale = 'en' WHERE locale = 'en_us';
UPDATE craft_elements_i18n       SET locale = 'en' WHERE locale = 'en_us';
UPDATE craft_entryversions       SET locale = 'en' WHERE locale = 'en_us';
UPDATE craft_sections_i18n       SET locale = 'en' WHERE locale = 'en_us';


Edit permissions:

UPDATE craft_userpermissions SET name = 'editlocale:en' WHERE name = 'editlocale:en_us';


Check:

To be sure you've got them all do a sql search for en_us.