The new conforming limit will be determined later this month... it's unlikely it will decrease but not guaranteed to increase. The CAMB (California Association of Mortgage Brokers) is lobbying pretty hard to increase the limit due to California's high median home price (~$550k).
It means that more loan programs will be available for those individuals whose financed loan amounts are higher than $417k (current conforming loan limit) up to the new conforming limit. This primarily affects what you refer to as "prime" credit, but also Alt-A & sub-prime markets are somewhat sensitive to that loan limit as well.
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