I want to talk about two resources in two different environments: Experience and Gold in Team Games and in "Free for All" games.

First Gold.

There is an upgrade that cost 2000 gold which lets that gold mine get 250 extra gold per turn (meaning it takes 9 turns to start profiting). Does this upgrade have to be reinvested if you let an ally in a team game take over it for a few turns? Do you even get the chance to upgrade the mine if someone else takes it over and you take it back?

Experience.

It might seem like experience isn't something you can "share" with anyone, but it is. In team games I was told you aren't able to attack people on your team, but in "free for all" games you can send units to someone as a sort of "food" to increase the exp of some of their units (and you can possibly work out for them to do the same). How often is this tactic used and is it acceptable behavior? Its not really a "bug" but maybe some people wouldn't like this behavior because it could be seen as helping double team against somebody (what is the controversy with that what is the general feeling on it?). A situation I can see where it might be a little more acceptable would be if I was trying to negotiate a NAP and had to bring this tactic to the table as a sort of "bargaining chip". Like feeding the dragon a small amount villagers else he devours the whole village.

Hi Bahlum,
As far as the gold, the upgrade stays with the goldmine once it is paid for. Whoever takes that mine gets the extra production, no matter how many times it is captured. The only exception being if the mine is destroyed. I'm not positive, but I think it goes away then and must be paid for again. I have not seen this happen, so I don't know for sure.
As to the experience question...IMO it would be fine. If you have so many extra troops running around that you can afford to feed them to one of your neighbors, it would be all the better for me when I come in from the other side to take out your castle and mines.
All the best,
H.

I can verify that destroying a goldmine will cause it to lose its upgrade. If you retake it later, however, you will be able to upgrade it again.