In game exploit.How does it work?

You can get cash, but you will lose residents

- meaning less tax

I'll try it and let you know, if its worthwhile

If you built the buildings before your land was upgraded, then yes you have to destroy them to get the modern looking skyscrapers. I have been demolishing a few at at time to keep my population from going too low. I also added services and have not grown my city as much lately to try to get all the previous buildings to turn into modern skyscrapers. The cash you get from tearing them down is minimal though. 

It works,but it is very monotonous.   In about 4 days I ground out 500,000 on top of purchasing all the buildings I needed.  I had a lot of spare time, so it may take longer if you have a life.  Keep in mind you won't be able to expand your storage or land very much due to the market being what it is.   I've spent more time attempting to buy items there than I spent grinding out cash.