The current pricing was influenced by yourself buying your ticket.
That means it makes no sense from the math alone to allow you to change.
That could be mathematically overcome if you pay extra to make that change possible. Your new price would not be much lower in total.
It is even possible that your ticket is cheaper now only because you bought it:
Imagine there are two seats left.
You book a seat.
The airline knows that the flight is booked mainly by couples.
The risk not to sell the single seat is great, so the price is reduced.
If you had not booked your flight, a pair of seats for a couple would be still available, and no reason for lowering the price would exist.
You caused the low price yourself by buying a ticket earlier.
The example shows that it does not make sense on the level of math alone, so you should not expect to get the cheaper ticket for a lower priceprice¹. The lower price is an offer to people who do not yet have a ticket, not for you.*²
(This does not strictly mean that you can not get the reduced ticket, because companies do not always behave rational.)
*)¹ The example is contrived and only applies to the last two seats on the plane. As a general rule, the opposite is true: Last-minute prices are the lower, the fewer tickets have been sold. Buying early drives late prices up.
²(It would be ethically controversial, but there may be a way to cheat that system.)