adsense question for new site

I just bought a new site on flippa the other day and the seller entered my adsense id into the website for me, i have checked this and it has been done.
the seller told me i don't need to do anything in my adsense account as google will automatically see that the id has changed.
Is this true?
also
if true how long does it take for google to make the changes in my account..

Thankyou

See original post by mathewaki