Since most people renew insurance on a calendar basis, very few will lose anything immediately. It's when they go to renew next year that the **** hits the fan and in the interim the Republicans will scramble to get something and blame their inability to do something on Democrats and they will be believed.
One of the most important provisions of the ACA was the end of "rescission," the previous practice where an insurer could look for an excuse to cancel a policy if the owner started making big claims or just was diagnosed with an expensive illness. If that prohibition ends, insurers could start canceling expensive policyholders the next day.
http://www.healthcare.gov/glossary/rescission/