Sure. How much data do you want a month?
For an easier option (getting through to Optus now is very difficult) I would suggest you just sign up for a Boost $150 Prepaid SIM. Works out a $12.50 a month for 7Gb each month (use over 1 or 12 months as you want). Or the $300 plan gives you 20Gb a month for $25 over the year.
The advantage of this approach is its largely automated. You just note you want your phone number ported over and it will be applied to your new SIM. The Optus account will be closed automatically.
Later in the year you can perhaps look to go back to Optus with similar prepaid deals (but they aren't available to existing customers like you atm)
PS To answer your question directly though, Yes all Optus plans are now month to month. So long as you're not paying off a new phone then you can pick any plan you want any time and switch to it. Cheapest is $39 for 10Gb a month at the moment.