But the next promised Windows 10 Phones build will be compatible with all Windows Phone 8.1 builds it seems. This is confirmed by Gabe Aul.
@Mxiden @ahmedbmali @kalintzis Yes, next build will support upgrade from higher base versions.
— Gabriel Aul (@GabeAul) March 31, 2015
You can catch all Windows 10 for Phones details covered on our dedicated page or by checking individual articles by clicking here.