Can you take over AI races (and vice versa)?

[dmm]

I seem to recall posters mentioning that they took over an AI race to see what it was doing. Am I imagining things? If not, how does one do that? Also, can the AI take over a player race?

I'm asking because in some games when I build up an early lead, it might be fun to switch sides to see if I can rescue the poor suckers who've been suffering under such bad leaders. Like those "wunderkind" CEOs who go around rescuing companies (supposedly).