A Sativa is a Sativa, an Indica is an Indica, and a hybrid is a hybrid. Harvesting them pre-mature isn't going to change these facts.
A sativa is a sativa and an indica is an indica and a hybrid is a hybrid but you can slightly fine tune the high each one gets according to when you harvest, and harvesting them late will not increase the level of THC.
It will decrease levels of THC through oxidation, something that can be visibly seen occurring as resin-heads turn cloudy and amber. When that begins THC ages and breaks down, this process is known as oxidization. High levels of CBN tend to make the user feel messed up rather than high. Most people confuse increased levels of CBN for higher levels of CBD.
As I have said for years, if someone wants a body stone, a couch-lock effect, pick genetics that are high in THC and high in CBD but low in CBN and then do not harvest them to late, as in when amber when you have lost THC to oxidation.
A sativa, a true sativa and not the various crosses most people consider to be sativas today, run ahead of the standard charts for maturity. A sativa with mostly clear trichome-heads with an every so slight beginning of cloudy has hit its maximum levels of THC and is already beginning to lost THC through oxidation. I say clear but some true sativas will not have totally clear trichomes at any point, they will begin as slightly yellow.
Gland color will vary with ripeness of the individual THC glands, starting at clear, turning to milky, then turning to an amber color. There may be some exceptions, such as some of the Blue strains and Blackberry. These will have darker, sometimes purple gland heads. But by far and large, these three steps of ripeness will exist with the vast majority of strains on the market.
The first stage, clear, will tend to make for a more cerebral, up high, with very little body effect. Harvesting at this stage when little if any trichomes are milky yet, can be tricky. Harvesting too early can make for a beautiful plant that will not have much psychoactive capability! THC needs to "mature" to be able to pass its psychoactive capabilities down to the smoker. This is why I always recommend waiting until 50% of the trichomes hit the second stage: milky or creamy. At this point, you will definitely have a plant which has reached its genetic potential.
The milky trichomes have some "cerebral" qualities, as well as some of the more "physical" characteristics of the high. The last stage, amber, brings on a more sedative, physical stone. If this is what you're looking for, than I would recommend a sativa/indica cross, or an indica dominant plant. Amber trichomes seem to ruin a sativa high if the plant goes too long.
Many times you'll read from new growers, "My hairs are 50% changed to brown or red, looks like its ready to come down." Calyx hairs turning color is one thing, calyx hairs withering, is another. When they have withered, they are for the most part, as ripe as they will get. But just turning red/brown does not necessarily mean the plant is ready. Calyx hairs are just an indicator that the plant is ripening, not necessarily ripe yet. I have taken full Indica strains with 60% hairs turned, and sativas when 90% hairs have turned, and they were all at peak ripeness. The trichome color is the only true indicator!
When using a magnifying glass or loupe to determine ripeness, youre examining the resin glands on the buds for signs of maturity and/or decay. During flowering, these glands start out small and increase in size as they fill with resin. As they near maturity, the head of the gland will begin to expand and it will start to look like a tiny mushroom.
This is the first sign that theyre nearing the point of diminishing returns. This is the point at which production of fresh THC laden resin has slowed and is equal to the rate of decay of THC into CBN.
The decay of THC is accompanied by a change in the color of the resin from clear to amber. This is the other sign of ripeness that can only be determined by magnification. What youre looking for is the point at which the production of fresh resin is still just barely outpacing the decay.
You said; "A Sativa is a Sativa, an Indica is an Indica, and a hybrid is a hybrid. Harvesting them pre-mature isn't going to change these facts." Well that is not totally true, people do it every day in reverse by following advice like you give, let it mature more so harvest late .... and the result or doing so alter their product. It works the same either way but you wrongly call harvesting at the prime time harvesting early.
Allowing a plant to flower longer is not the only way to assure higher levels of psychoactive THC. Regardless of trichome color a fair amount of THC is not in a psychoactive form at harvest. That is why curing is done, to allow all the THC to become fully psychoactive.
You do not have to flower plants to long and lose THC through oxidation to achieve fully psychoactive THC. Even then not all THC would be psychoactive, a cure would still be needed to make the most of the then lesser amount of THC plants would have if flowered to long.
For all those who were unlucky enough to come along after the age of real pot, the true sativa age, and were fooled into thinking that indicas/predominantly indicas are great, well then if that is what you like purchase genetics with high levels of THC and CBD and low levels of CBN and do not flower to long and give up THC for CBN thinking that you are getting higher levels of both THC and CBD.
It just doesn't work that way.