By changing your plan from a Medium to Small tab, you'd be asked to pay for the whole remaining device balance. That device balance is the phone subsidy you have activated with your Medium plan when you got, order or activated a new phone with us.
If you're looking to modify your current plan to have different features (more calling-minutes or data for example), you will need to review your options within the same tab (Medium).
Hope this put a little bit more clarity into the matter. Cheers!