Well... when you decarboxilate the weed (heating or curing properly) the THCA is converted to THC.
Robert A. Nelson: Hemp Husbandry ~ Cannabinoid Chemistry (Ch 6) ~ Extraction

When you isomerize the hash oil you convert CBD to THC.
Robert A. Nelson: Hemp Husbandry ~ Cannabinoid Chemistry (Ch 6) ~ Isomerization

But the exact amount which is converted depends on several variables... time of heating, temperature, degree of purity... so, i think the best way is experiment (smoking, of course :stoned until you find the exact proportion CBD/THC you want.