Hey @monykeo00 - Great question to ask!
Depending on the phone type and model, the cost will vary. It's something you'll need to speak to our Messaging team about > https://app.optus.com.au/msg-us?tags=OCAYCMSG whom can confirm the exact cost for you. Changing over from a Lease plan and retaining the phone isn't a long process though.
If you're based outside Victoria, our teams in store can also assist with this - Steven