This is inconsistent with the spell itself works. If you cast Doom on level 1, and then level it up to level 2, the already cast Doom will not update and continue use the level 1 damage. This makes sense, since it was cast on level 1.

However, the talent damage updates all current instances of Doom, even when they were cast before the talent was chosen. This should not happen.

The entire damage should be set upon cast, and not update afterwards.