Zoning changes happen all the time. I'd guess thousands happen every year in this city. You can see the list that gets voted on by city council and it's pretty long every month.
You may be thinking of a Planned Development which is a negotiated zoning change between developers and the city for larger/more important projects.
Once a PD is in place it's harder (but not impossible) to change zoning within it.
I'd guess this tower sits within a PD, but the lot next to it may or may not be included.
Here's the ordinace that outlines all details of this PD plus all (yes zoning changes) to it since it was created:
https://gisapps.cityofchicago.org/gi..._pds/PD499.pdf