Well our mouths wouldn't change just because they are no longer needed the way they were before.
It would only change if there was a negative effect of having a mouth and teeth like we have now.

You could say if we don't need our teeth anymore, they might disappear over time, but that would only occur if having teeth lead to higher death rates and fewer children being born. But we've generally come so far with our technology that we're not really that much dependend on the "survival of the fittest" rule anymore.

If we didn't need our teeth and let those people who have tooth decay die from infections instead of treating them with our modern technology, then very very slowly those who had a random mutation that leads to them not having teeth at all actually have a better survivability than those who have teeth. Buit as we treat them, they have the same chance of reproducing as everyone else.

You could also argue that it's better for survivability to have as few body parts on you as possible that you need to provide energy for and those parts that you have being most energy efficient. So it could be that we at least develop smaller mouths when it's no longer a benefit of having a bigger jaw etc. But this also wouldn't happen because there is no reason why someone would die of having to spend too much energy on their body nowadays.