You can change back fairly easily @dkk . . . you might have already.
I know, you shouldn't have to, it should be offered, and if anyone wants to go monthly then they can opt in.
It is what it is, as you say the cashflow improvement would be great for AGL, and likely they might have fewer difficult larger bills to collect when things do go to recovery.
For what it's worth, this has happened to me a few times in recent years, I just change back as I prefer paying 4 bills a year rather than 12.
If you need some guidance to change back . . .
Website account online . . .
Go to Billing tab
Click ALL PAYMENT OPTIONS in the payment options window
Click bill frequency monthly
Click edit
Click to change to (stay on) quarterly.
Phone app . . .
Go to account tab at bottom right of screen, click settings
Under billing and payments, click monthly billing
Click relative account, and change back to quarterly