This time, since I hadn't exchanged money yet, I needed to convert Japanese yen to Chinese yuan.
Literal
This-time, still exchanged-not-had [because-ので], Japanese-yen [object-を] Chinese-yuan [to-に] exchange need existed.
まだ~していなかった is the past negative of an ongoing state: 'had not yet exchanged.' 両替 (money exchange) is a common travel term. 人民元 (renminbi/yuan) is the formal name for Chinese currency. 替える (to exchange/convert) takes を for what you're converting and に for what you're converting to.