THC A -> THC

The trichomes and the THC they contain do degrade and oxidize over time. This is why you cure and store in a sealed, dark container to minimize the decay. So, you are correct that it does change over time just not the "high" portion. The conversion to a psychoactive form is largely a product of temperature and how long it is held at that high temperature. Most information I have read lists 80c (176f) as the lower end for this conversion. This is why you heat (decarboxylate) the weed before making edibles. Even cooking doesn't heat it hot enough for long enough time so it is done as a separate step. Smoking and vaporizers work by heating the flower. The intense heat of burning almost immediately makes the conversion while in a vaporizer at a low temperature may take a few seconds. Curing is a totally different process and does not significantly affect the psychoactivity. It can have a huge impact on flavor, aroma and long term storability... but not the high.