My wife actually decided to try this last night... she had a couple hundred grand and was not pleased with her initial zoning decisions. She paused the time, and flattened the city, leaving only a few high density streets (not avenues), her fully upgraded sewage treatment plant, and a well upgraded water treatment plant.
She then turned the remaining plants off, and placed trees over all of the polluted areas.
She put it in Cheetah speed, and waited for the population to disipate.
When the pollution faded to what she deemed to be an acceptable level, she began re-zoning the city and placing services according to her master plan. The population exploded rather quicky, and the leftover roads were immediately supporting brand new high density buildings...
As a note - My connected city took about half of her population when she did this.