ok I don't know about anywhere else but in Northern Ireland they change techniques every so often usually for the better, but one recent change I've been thinking about: Shortning long stance ie. becoming higher up. I don't think this is such a wise choice as my physics course in school says "the lower the centre of mass the more stable it is". Does this not make long stance less effective in balence? or easier to push over?

The height of the stance involves finding a balance between stability and mobility. You will always be sacrificing one for the other. However, in internal martial arts, they train to sink the qi the which enables them to maintain a very stable stance without having to go too low.
Generally, the more stance training you do the more stable your stances will be so you may be able to afford, in combat, a higher stance so that you can move more quickly from one position to another. Often in combat speed and unpredictability are more important than a low stable stance that gives you the sometimes false comfort of strength and stability.

Nice post, puffadder. Welcome to the forums.

Gotta go with what puffdadder said.

Ich bin curious, which TKD association have you been getraining with? Have these changes been just local adaptions or across a wider (inter)national syllabus?

Taekwondo, being perceived (and promoted as well as trained) as primarily a striking art (read "kicking), the stress would of course be on mobility and speed. That would necessitate higher and shorter stances. Which makes sense, since long, wide stances would be detrimental to one's game.

Same goes for boxing and kick-boxing games. Getting into the ring with a boxer and using a wide, long straddling (horse) stance is a good recipe for getting your lights punched out.

Then again, stances may not be as they seem. Those long, wide stances are not so much for footwork/mobility as they are for unbalancing, tripping, throwing, sweeping, displacing, etc., which, of course is seldom, if ever, taught in Taekwondo nowadays.

BTW, the way I see it, the horse stance of Taekwondo (the WTF variety) has actually changed to a "thin-camel stance" or a llama stance. Or else, the horses are getting thinner and thinner.