It suited both sides for the transition to be gradual.

For the UK there was concern that if the PLA arrived in force in Hong Kong overnight to make it part of China in practice as well as in law many, perhaps millions, would try to defy them and demand sanctuary from the British. The international community, looking on, would certainly prefer the story "This is Britain's responsibility" over "We all need to take some of these people". So then either the UK has to find somewhere to put a LOT of people or it is seen to have failed badly as the Chinese put down the resistance.

For China it was recognition that their notional sovereignty over all of "China" must be temporarily shadowed by the consideration that Hong Kong is not just some nice waterfront property (which they could have seized at any time by military force) it's a people, a culture, lots of successful businesses. If you roll in with tanks overnight that probably goes away and you might not get it back.

But gradual is the thing. "Unchanged for a period of 50 years" is merely the ambition on paper. Nobody in Hong Kong thought they'd wake up in 2046 still under the same democracy they'd had under the British. Things get chipped away, they shade over, decisions are made that might once have been different, this year's protests are about some of the less subtle changes.

Gradual change suited both parties. Not the people of Hong Kong themselves of course, but they weren't signatories.