I was wanting to know if at the end of a 24 month contract, you were able to stay on the same plan but renew the contract with a new phone, I like my plan so don’t want to change it but need a new phone
That should certainly be doable. You should be a ble to pick the handset you want and they'll contract you for another 24 months with the extra amount needed to pay it off.
FWIW There's little $ benefit in monthly installments over outright these days (and in fact outright is usually a bit cheaper). If you can afford it then IMO that's the way to go. Also you get the benefit of being able to see the occassional spot sales Optus (or another Telco) has and jumping on them if you want.
You can also find the same phone on Ebay for about 60% of the price. Make sure you buy from a 99.8% reputation seller, not 96%.
However, it's quite likely that when you look at the price of the new phone vs. going on a new cheap contract, you may pick another cheap contract. Just wait around for some bargain to pounce on.