Whether or not you can switch from a fixed rate to a variable, and how much that could cost, is really dependent on the terms of the loan you have with your lender. It would be best to familiarise yourself with these terms before choosing to make the switch to make sure you don't get stung with any fees or charges.
An Aussie Broker can help you compare home loans that allow you to switch between the two types of rates, and also understand any costs and processes involved in doing so. This service is part of your free appointment with an expert Aussie broker.
If you have an AHL product then please call customer service on 13 13 33. They will go through the details of costs if there are any and which will need to be paid prior to the switch taking place.