All journeys you made before you change your season ticket will be charged as Dal Voordeel. All journeys you will make after you change your season ticket will be charged as Dal Vrij.
If you change your season ticket during the month, you will be reimbursed for the costs of the old season ticket for the remainder of the month and you will receive an intermediate invoice for the new season ticket for the remainder of the month. In your case, for example, if you were to change to Dal Vrij today, this means you will receive back a few euros for your old subscription and pay about one third of the monthly costs for Dal Vrij for the remainder of this month. (Here you should read ‘the remainder of the month’ as ‘the period until your next regular invoice arrives on the 6th’.)