Since she can change a friendly ship's "top command dial", does this mean she could change the top command dial of a ship that has already activated? Which would essentially change their command dial for the next turn?

I'll really need to re-read the card, but at first blush, as long as the only parameters for selecting a target for her ability is that it is a friendly ship, and is in a certain range (which I think it indeed is), then yes, this seems valid.

It's the top dial so since an activated ship has used it's dial the next one down is the top dial.

I think that this gives Leia Organa even more utility. You can use her to change the upcoming (next turn's) command dial for a ship and then, next turn have her standby with an alternate command in case your first plan runs into something unforeseen.

never thought about changing upcoming turn that is good idea

It has popped into my head a few times while using her but the only time I can think of that being needed is if my thoughts on how a squadron engagement went wrong and I need more fighters in or I need to finish it early, or I need a repair command.

The issue with that is now you are no longer planning for this turn to swap but next turn. Things get tricky then

From the wording on the card the friendly ship you are targeting would have to have been activated already. Other than that I see no issue with it. I'm also now far more afraid of Leia. Nebulons and AFIIs just got a bit more scary.

Well Leia can hit unactivacted ships as well. It let's you respond to the threat profile of the turn.