It seems you have a fundamental misunderstanding. China had been using the metric system for decades prior to 1976, it just wasn't the only measuring system in use. Since the early 20th century China had been using the metric system alongside other measures, and they progressively moved closer and closer to the metric system in steps up through 1984, when they finally decided to curtail the use of other systems (1976 was when Hong Kong adopted metric, not China).