Something like this maybe:
UPDATE Buildings SET PrereqDistrict = 'DISTRICT_CITY_CENTER';
UPDATE Buildings SET RequiresPlacement = 0;
Do this for each type of district and buildings should be fine. Also
UPDATE Districts SET PrereqTech = 'TECH_FUTURE_TECH' WHERE RequiresPlacement = 1;
I know some districts have civic prereqs, but since buildings can have both tech and civic as a prerequisite, then districts should work as well, so it shouldn't allow them to be built until you get to future tech.
Note I do not know what this means for wonders with terrain or other location requirements, you may have to null or 0 those out as well.
Hope this helps, and good luck.