When the Chinese research the tech that enables them to turn their capital into a metropolis, does the metropolis start with walled side up or down?

There is no clear rule on this situation.

I would say that if a capital has walls when one converts it to a metropolis it keeps the walls. Wether it's chinese or not.

That would make sense actually. Why would you have to build walls twice just because you make your city bigger. (ignoring logic)

Ok. So maybe the city grows larger outside the walls but the important societal and governmental buildings are most probably still behind the original walls. And besides, I would think that the citizens from outside the walls quickly move inside the walls if a siege is on the horizon.