Here’s a data point. I have a reservation which the price dropped ~$50. The fare rules state fare difference must be refunded. I just made a flight change and JL asked for tax differences of $1 without refunding the $50 fare difference. My understanding is my fare basis didn’t change and it was an even exchange. So nothing is refunded.

I made the same change with AA for another passenger, it’s booked under the same fare basis and had the same price drop. AA made the change without refund nor ask for the tax differences because it’s an even exchange according to the agent.

So that fare rule clause probably only applies if it's not an even exchange.