I have an iPhone 8 on a 24 month (buying) plan with Optus with the $45 a month plan as well. The only thing is, this plan only has 4GB data a month... Is there a way I can upgrade to the 20GB plan?
Yes I believe you can go up a plan but you can never go down. Ask sales on the live chat about doing that.
Edit: The CIS for your plan says just that.