Nice, this will allow the world to develop in a much more natural way.
Will there be demotion though?
Can pops demote through famines (instead of just migrating away), sieges, raiding, unhappiness, etc?
If not, I can see, in the lategame, most of the pops in the region the nation starts on plateauing into a population composed entirely of citizens.
Or is that the intended behavior?
You can still manually move slaves as you like between cities, for a price in gold, but the UI have been completely changed, so instead of selecting a pop then a target, you instead go to the city you want slaves in, and then select what type of pop you want, and its origin and click for as many as you want in the same UI.
Will they still move instantaneously though?
I don't think it should take years, or even more than a few months, but teleporting pops from one end of the map to the other doesn't feel ideal either.
Conversion is 1 pop at a time.
Will number of pops (or lack thereof) affect speed as well?
I can imagine that if it doesn't, conversion will be done much faster on pops spread through a large area than where they are more concentrated.
Not sure how much sense this makes history or gameplay-wise, but it's something to be aware of.