Data Plans

What if I am 2.35 GB over my 3GB plan and (being charged $45 plus the $30 for the plan) and  I opt to go to the 10GB plan for $60 , will that update my bill to the $60 for the 10GB plan instead of the $75 for 3 GB plus 2.35GB ?  or is there a pro rate ? The reason I ask is I have 2 days left in my cycle and if I can save $15 by switching to the 10 GB plan I will