It makes little sense to me that the NCR would let you switch the power distribution around at your whim since that sort of activity would be extremely noticable. I don't think you should be able to switch it at all. You should have to live with whatever decision you made (or reload an earlier save).
Err ...
In case you did not notice Helios One is not really much of a priority and they were only there because the Brotherhood of Steel was there, not because they really needed a power station as Hoover Dam generates more that enough power to the area, the Strip actually only gets 5% of the power generated by Hoover Dam.
Its a priority to the Followers for two reasons, it would prevent gaining access to pre-War weapon and would help Freeside (as House only cares about The Strip), the NCR really does not care that much as they have Hoover Dam and only annoys them a bit if you divert the power.
If for whatever reason Obsidian decides to make it possible for you to switch the distribution to another recipient, you should have major consequences as a result of that. For instance, if you change where the power is going, the NCR comes down on you hard, throwing dozens of well-armed soldiers at you. It should be a risky decision in itself to change the distribution (if God forbid they change the way it works), not some "Oh I think I'll give the power to X today!"
Why? they did not cared when you decided to divert power in the first place that much, only annoyed that "some tribal" screwed up Helios One power grid, its like you blown up Hoover Dam.
Besides what purpose would it made if there are "major consequences" of something that DID NOT had major consequences before, its not like the one responsible to fix the place is called "the idiot with sunglasses" and goes by the name of "Mr. Fantastic" that is a clear indication of how strategic important it was for the NCR, its such a important place THE COMMANDING OFFICER WANTS TO LEAVE!
I understand wanting to divert power later on since its really a arbitrary decision that prevents the player of doing so and not a logical one but it that is a decision you make at the end of the quest and allowing the player to change it at will would simply be not giving a consequences to the choice the player have to make and one the player is informed if the player look around the place and talk to the NPCs.