Usage of のに in this sentence: じゃ、あたらしいのにかえましょう

Question

Assuming the following sentence and its answer:

にもつがおもくて、ふくろのひもがきれそうです。
じゃ、あたらしいのにかえましょう。

I don't really understand why we have to use のに instead of を in the answer, actually I'd have answered like this:

じゃ、あたらしいをかえましょう。which would mean "Then let's buy a new one".

The way I interpreted the usage of のに is more like "Then we'll have to buy a new one." Is that correct or is there a more accurate translation?

Answer 1

あたらしいをかえましょう is ungrammatical. あたらしい is an adjective, and the object of a sentence (thing marked by を) must be a noun or noun phrase.

あたらしいのをかえましょう would be grammatical but it would not have the correct meaning. The の here means 'one' and makes the adjective into a noun phrase. This sentence would mean "Let's change the new one". But, you want to change to the new one, hence に rather than を.