When will Windows 10 be available on the Lumia 1020?

[Steve Adams]

Its telling you something....ha ha.

 

[dzuidema]

But previously with some insider updates you had to hard reset your phone. It reverted to win8.1 and then you could update to the newer win10 build.

 

[Chintan Gohel]

But previously with some insider updates you had to hard reset your phone. It reverted to win8.1 and then you could update to the newer win10 build.

that might have been a long time ago. Hard reset wipes out personal and app data and installs that current OS afresh without any baggage from a previous installation

 

[charlatan1978]

But previously with some insider updates you had to hard reset your phone. It reverted to win8.1

The only way to go from Win10m back to WP8.1 is the 'Windows Device Recovery Tool' (WDRT).
You are right about one of the older builds, where it wouldn't upgrade from the previous build.
On that occasion it was recommended to roll back to WP8.1 using the WDRT, before upgrading.