Changing my plan but they say it doesnt meet the terms of contract?

[JayToronto]

Hi, i am using iphone4 which ive been using for almost 2 years out of 3year contract.

I am currently using 50$ rate plan including 200min 500mb.

I ve just noticed that there is lite voice&data plan which costs me 40$ for 150 min and 100mb.

I dont usually use up the airtime minute and also i dont use data that much( it says 10$/500mb so I think

the total bill will be cheaper anyway even if i go over 100mb)

So i decided to choose this lite plan and when I clicked on the continue button, website says 'your change

doesnt meet the term of contract, one time fee will apply"

I dont get what this means? can someone explain about this please?

[WorldIRC]

You must maintain a $50 minimum voice and data on the account.