I like this idea as an optional extra, not as a default setting you can't get out of.

It is nice to play in a team with someone you know, but additionally it would sometimes be quite fun to be assigned a random team

I agree with rathmaskalion. If you know that your target will be switching after, say, 10 rounds, then you will simply need to adopt a new strategy in anticipation of this (ie. don't take your target down to 1 territory on round 9!)

It also introduces the element of surprise, which is lacking in ...

I love this idea