You will need to Optus sales but it is very likely they will let you upgrade your plan to lock you in for another contract.
Before you do check on what is available with all telcos. It may be better to wait a couple of weeks and save money over the next couple of years. Saying that Optus may well offer the best deal for you.
Agree with @Davelew,
Coming off plan is a bit liberating. You can also pick any SIM only deal if you wait a bit and see what comes up. Price is easily half what you are paying now. But if you want a new phone then Optus will almost certainly facilitate. Current plans also offer a upgrade to new again after 12 months for $99 which might be to your liking.