The real impact of the Republican AHCA will be on the working poor who now qualify under Medicaid expansion but won’t with the expansion eliminated. Those earning a little more, but not quite eligible for Medicaid, will also be screwed by the elimination of the Obamacare subsidies for purchasing health insurance.
The Republican bill will also allow much higher premiums for the elderly.
As to preexisting conditions, it may be an unintended effect of the bill, but as it is written now it will greatly increase premiums for those with preexisting conditions. It’s rather complex how it will indirectly do this, but is explained here: https://www.brookings.edu/blog/up-front/2017/04/27/new-amendment-to-gop-health-bill-effectively-allows-full-elimination-of-community-rating-exposing-sick-to-higher-premiums/
Another technical glitch to this bill is that it would eliminate the healthcare tax credit for up to 7 million veterans. http://taskandpurpose.com/congress-veterans-ahca-trumpcare/