To " trade in" means to exchange one thing for another thing.

。 “以旧换新” 是指用一种东西换取另一种东西。

" To switch" means to replace or exchange one thing for another, to buy something new in this case.

“To switch” 是指用一件东西替换或交换另一件东西，在这种情况下是买新东西。

It is the necessary, though very slow and gradual, consequence of a certain propensity in human nature, which has in view no such extensive utility; the propensity to truck, barter, and exchange one thing for another.

它是人性中某种倾向的必然结果， 虽然非常缓慢和渐进， 但这种倾向并没有考虑到如此广泛的用途； 卡车、易货贸易和用一种东西交换另一种东西的倾向。