Starting from March 2019, is it necessary to build all existing apps using iOS 12 SDK and Xcode 10 for it to work on iOS 12 devices? This is what the apple developer website says.


  Starting March 2019, all new apps and app updates for iPhone, including universal apps, will need to be built with the iOS 12 SDK and support iPhone XS Max. All new apps and app updates for Apple Watch will need to be built with the watchOS 5 SDK and support Apple Watch Series 4


Does it mean that even the apps already available on Apple App Store needs to be resubmitted with iOS 12 SDK build for it to work on iOS 12 devices starting march 2019?

And what about enterprise apps that are distributed as .ipa files? Will this also need to be rebuilt with iOS 12 SDK for it to work on iOS 12 devices?
The key words in this announcement are "new apps and app updates".

Apps which are already available on the App Store do not need to be rebuilt with the new SDK. Apps which were built with earlier SDKs will remain available on the App Store, and will still be usable on devices. However, after March 2019, the developers of those apps will need to rebuild their apps with the iOS 12 SDK to release any further updates.