The CBO estimate is not 800K job losses, it is 800K people leaving the labor force because they are only working to help pay for health care and would be covered by increased Medicaid coverage. From the CBO:
The expansion of Medicaid and the availability of subsidies through the exchanges will effectively increase beneficiariesí financial resources. Those additional resources will encourage some people to work fewer hours or to withdraw from the labor market. See "The Budget and Economic Outlook: an Update here
It is being implemented little by little, but the state exchanges and individual mandate are key components and become effective 01/2014. More than two years is enough time for major constituencies to propose improvements to the AHCA that address shortcomings. Whether they choose to do so is unclear, but this seems to be moving away from a binary "pro v con" view to a more pragmatic attitude of implementation.