Yes you can, you're already finish with your contract so basically the phone is yours now. You can freely move to prepaid. 😄
My answers are just my opinions. It doesn't represent Optus or any of it's staffs.
Click Kudos if you find it helpful, and please mark it as an anwer if your problem has been resolved! 😉
Just switch to a monthly no SIM plan until you decide what to do. If you do nothing your existing plan will just continue as-is.
$50pm will get you 6GB data. If you want to keep your phone for the next12 months, you can get $40pm and 12gb.