Changing mobile numbers and porting are very different. If you have say 0411 222 333 and want a new number which becomes 0422 333 444 then you'll be charged $55. Optus will usually waive this for you if you're changing the number due to something like nuisance calls.
Porting your number is when you transfer your existing number from one carrier to another, say Telstra to Optus. There is no charge for this but you're keeping the same number, not changing it.