xap will be for windows phone 8 or windows phone 7 devices with old API and APPX or whatever the new file format is called will be only available for windows phone 8.1 devices since Microsoft will introduce many new API for developers ; another change is=they will merge windows phone 8 apps with windows 8 apps ; just like apple did for iPhone and iPad ; developers will have option to create apps for both windows phone 8.1 and windows 8.1.