Apple confirmed in its internal document sent to the retail stores and Authorized Service Providers on April 27th that “service inventory of all iPhone replacement batteries is now available without delay” (Via MacRumors)
This means that Apple Stores and the Apple Authorized Service Provider can directly replace the battery from Apple orders instead of waiting for the batteries to come. However, it does not necessarily mean that each retail store or authorized partners to provide battery replacement service immediately.
The details in Apple’s internal document may be valid globally, but it may subject to change from region to region, As the Apple Support page still says the process may take up to 5 business days. Still, compared to before users may wait several weeks to get it replaced, this is a huge improvement. You can receive battery replacement before the end of 2018, so you are no need to be rush.
Image Credit PhoneDog