It won’t be a complete replacement, not even close. It will be a gradual takeover of routes as aircraft become available.
Brisbane will be the first base as an all economy Jet can do Cairns and Isa with no-one even blinking.
Even a 20-30% takeover of 737 routes is a huge cost saving to the company. It also provides protection against industrial issues (PIA) as they can keep the punters moving.