Unfortunately once you have a plan with Public Mobile you aren't automatically eligible for all of the lower plans posted anymore. You can take a look at what plans are available by logging into your account > Subscriptions & Add Ons > Change Subscription then having a look at what is available specifically for your account.
Some users have been successful in doing what is called Trickle Down Downgrading where each new billing cycle they choose a slightly lower plan until ultimately being able to select the plan that they had originally wanted (I believe you can normally only do a slight downgrade each month rather than one large jump). I haven't done that myself but have read of people being successful doing so.
Hope that helps!
