Yes, you will pay the season ticket costs for the next month (6th November until 5th December) and the travel costs of the past month at the same time. For these costs you will receive an invoice around the 6th of November. (A little less than the 257.90 euros you mention if the 150 euros include the season ticket costs for Dal Voordeel for this month, seeing as you paid those costs in advance.)
If you change your season ticket today, then you will also receive an intermediate invoice in a few days, as I mentioned, for the season ticket costs for the remainder of this month (25th October until 5th November) of about 30 to 40 euros (since there's about one-third of the month still to go until the 6th, with a few euros subtracted as reimbursement for the Dal Voordeel season ticket you paid too much for in advance).