So far there have still been very few articles in the MSM describing exactly how the new healthcare system would work and what exactly it does.  From what I'm able to piece together your cancer patient example would be handled the following way:

Cancer pateint A buys insurance from company B.  They cannot be denied coverage and cannot be charged extra for their pre-existing condition.  Company B's aggregate risk goes up and thus their cost go up. 

This cost increase is dealt with in 2 ways.  First the mandate within the bill increases the number of people in the insurance pool, spreading the risk over more people paying premiums.  The second way is there is a mechanism for risk adjustment (I don't think this is the right term, but I forget what it is).  Basically it works sort of like the luxury tax in baseball.  All insurance companies pay into a risk pool and those funds are redistributed based upon the risk profile of the different individual companies.  Thus companies that have mostly low risk patients pay more into the system than they get back.  Companies with more high risk individuals will get more money back than they pay in.  It is supposed to be a way to remove or at least decrease the incentive for insurance companies to deny care to unhealthy individuals.