Not too sure about HKU B.Landscape Architecture course but if you do not want to take the 4 years Bachelor course there is another way to do it by appplying for the master course.
Personally I justed graduated from B.LA course and studying Master of Architecture oversea. Some university do offer ppl with Master from a non-related field but need to study for 3 years.
Given that u have 1.5year left i sugguest u to not waste anytime in switching bachelor degrees. Should look for uni that accept appliaction to 3 years MLA course and u should start taking some drawing / design subjects (basic) from your ARTS degree - start building up ur portfoilo since it'll become very handy to ur future application.
