How to change to a multi-month Prepaid plan

I'm currently on a regular $50.00/month prepaid plan. With multi-line discounts for 3 phones, it's not bad at all. But I figured out I can save over $250/year by prepaying for 12 months, while getting a better plan for one of the phones.
But in my account, I can see no way to switch over from the regular prepaid plan to a multi-month prepaid plan.
(I realize I have to split off the lines to make them individual accounts, but I would be still be saving $250/year.)

I would really appreciate some help on this.
Thanks much in advance!