а точнее вот -
I still have a balance on my OV-chipkaart. What will happen to it?
If you make the switch to NS Flex with your current OV-chipkaart, we’ll automatically settle the remaining balance on your NS Flex invoice. You won’t be needing a balance in the future, because NS Flex also lets you travel by bus, tram and metro on account. Smooth service, if we may say so ourselves.
это кстати в моем случае не так сработало. приходилось идти ногами и снимать оставшуюся сумму кэшем