Certainly looks like you can. I just logged into My Account, scrolled down to my mobile service, selected "plan info" on the right and could see the change plan button
Hey @nodata, as @sam Sam, suggested; simply log into your Optus.com.au/myacount and navigate to the number that you're interesting in upgrading. You'll be presented with a drop down menu on the right hand side of your dashboard. Click on the My Plan tab and you'll see the screen @sam Sam has screen shot above.
It's always a good idea to check whether or not a rate plan change fee applies. It's also important to note that once you upgrade your plan to a higher plan tier, you're unable to go back down. There's also a handy link here → https://offer.optus.com.au/mobilerpc/.