
I agree. I think 'In' introduces an observation by the writer; it
was a departure from tradition. 'As' introduces something intentional - as you said, a choice. The writer is doing a bit more than just observe - they are saying that Grandma made a decision.
Reverting to the original, 'In a sign of the times' is an observation; the use of cell phones
was a sign of the times. 'As' would be suitable if there was a decision, as in something like 'As a sign of our disgust at the increase, we will not buy any more'.
So 'As a sign of the times they used cell-phones' would mean that they chose to use cell-phones as a deliberate demonstration of their new attitude; it's possible, but I think the 'In...' version is better in most contexts. Ask yourself, did they choose to use cell-phones to demonstrate something? (As I said, it's possible, but I don't think it's likely).
Anyway, that's the way I see it.
b