The agreement between China and UK was for 50 years, so by right China can start changing things totally in 2047. So the issue is China is 28 years too early.
Why the downvote? The Wikipedia article even backs it up: "The Basic Law ensured Hong Kong will retain its capitalist economic system and own currency (the Hong Kong Dollar), legal system, legislative system, and people's rights and freedom for fifty years, as a special administrative region (SAR) of China." -- https://en.m.wikipedia.org/wiki/One_country,_two_systems
Why the downvote? The Wikipedia article even backs it up: "The Basic Law ensured Hong Kong will retain its capitalist economic system and own currency (the Hong Kong Dollar), legal system, legislative system, and people's rights and freedom for fifty years, as a special administrative region (SAR) of China." -- https://en.m.wikipedia.org/wiki/One_country,_two_systems