The first Windows 10 for Phones Build, Build 9941.12498 was incompatible with Windows Phone 8.1 builds higher than 8.10.12419.341. We have two Windows Phone 8.1 build higher than this in the wild: 8.10.14226.359 and 8.10.14234.375. In case your devices had these two higher builds you needed to use the new “Windows Phone recovery Tool” to re-flash your device to a lower build for updating to Windows 10 preview, even if you had a supported device.
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.