If, as part of changing the country targeting, one also decided to change the ad to match, then the ad would need to be reviewed/approved again.
A change to an approved ad, even if it is a very minor change, will bring the ad down from partner sites until it can be reviewed and approved. (Rememeber though, it goes live on Google very quickly.)
This is because changing a letter or two can completely change the meaning of a word.
Can I think of an example?
D**n right I can! ;0