Copy data from 1 field to another on same record. (either in Edit or Detail page using a weblink not

I need to create a weblink which can copy data from 1 field to another on the same record.

For example:
I am creating a new Account and I enter the Biling Address. Sometimes the Shipping address is the same and I do not want to enter it again.
Simply click on the link and it copies the data over from the Billing Address fields to the shipping address fields.

I have a part of the link that works (it copies billing and pastes it into shipping), however it does it from 1 record to a new record instead of the same record.