既然没有广东话课，我（便/转而/就/instead）选择学普通话了

既然没有粤语课程，我决定改为学普通话。I agree with all the other comments , besides those answers , 改为 also can be used here:)

First of all, Chinese do not use such a construct as like English. 'since there were no Cantonese classes, 既然沒有广东话课, I decided to learn Mandarin INSTEAD 我{就} 学普通话了. Here 就 means "thus". Sometimes a direct translation from the source language makes no sense in the target language.

我正在学中文，而不是中文粤语.

既然没有粤语课，我决定学习普通话课来代替（粤语）