I have bought websites from a person and he has google ads on there, can i jus replace the GOOGLE AD CLIENT number with mines and leave everything else there? Will that work?
Yes you can just change the ad_client "Pub- xxxxxxxxxxxxxxxx" - but have a look ad the channel code too, if there is one in the ads you want to change, you must delete it or create your own channels and replace the channel code too.
it would work, but it is technically against the TOS, which does not allow you to change the ad-code in anyway, period. that being said, i doubt that if you were only changing your PUBid that they would care. also watch the channels like Monfis said.. and remember: one typo, one extra character, etc may draw attention to the account and get you into trouble.
Yes, doing them again would be the best option. You can track them according to the channles you want. Considering the fact that Google only allows 4 ad units on a page, there should ideally be only 4 Google Adsense files being included in the pages, (if the code has not been directly inserted to all the pages.) So you should be quick to change the code for these 4 'include' files.