Changing owner details is not a problem. You would need to find out which database the dog is currently registered with as there are a number of national microchip registration databases in Australia. If you don't know which database, you can use this website to find out:
http://www.petaddress.com.au/Default.aspx
All of the registries require change of ownership details to be submitted in writing. Some of them have change of ownership forms available to download from their websites. All of the forms require the signature of previous owner and new owner as confirmation of ownership change. Alternatively, a stat dec can be submitted if you are unable to contact the previous owner.
In the case of puppies, in QLD they cannot be microchipped before 8 weeks of age but must be done by 12 weeks of age. Most of the breeders I deal with already have the new homes waiting well before the puppies are weaned so we can fill in the microchip form with the new owner details as soon as the chip is implanted.