Copy data from 1 field to another on the same record using web links

I am not very familiar with programming/scripting and need some help.

On the Accounts object the user enters the Billing Address and in case the Shipping address is same they do not want to type it in manually, instead a simple checkbox or weblink is required which would copy the address from the billing address fields to the shipping address fields.

This can be achieved using a worfklow with update value action and it works, however I would like to do this using a weblink.

With some assistance I have a somewhat faulty URL for the weblink.

The latter part of the URL (in RED) works like expected i.e. it copies the Billing fields and puts them in the Shipping address fields, HOWEVER it copies it onto a NEW RECORD page instead of the current record.