More than likely your tariff was changed the following day after you made the request. As a result of this, you were refunded the line rental you had paid on the previous rate and charged the new rate instead and then on your billing date you were charged for the follow month.
However if you were not on an unlimited minute plan to start off with, the minutes would have been adjusted for the number of days you were in your previous plan. As a result of this you may have gone over the minutes allocated for the number of days on your previous plan. For 19 days on your old plan you would have only got just under two thirds of the total number of minutes on your old plan.
