Consumer contracts don't guarantee GPON support in any way. So if KPN wants to upgrade they can just send the customer a letter telling them to get an XGS-PON compatible ONT by some date.
They'll probably take a bit more customer friendly approach and at least send you a free provider owned XGS-PON compatible one and a new modem. But for your own equipment you have to manage everything and make sure it complies with their published specifications.
That sounds like a somewhat pragmatic approach. Curious to see how that plays out in practice. I presume the total number of consumers that are interested in running their own ONT is limited. In Germany the situation seems a bit different. There customer owned Fritzbox devices with integrated ONTs are very widespread making the situation for an ISP quite different when it comes to upgrades.
Sure, but even with the fritzbox situation all ISPs also offer dedicated ONTs as free rental or for 40€ as purchase. Some, e.g. Telekom, also offer SFP ONTs (Digitalisierungsbox Glasfasermodem)
They'll probably take a bit more customer friendly approach and at least send you a free provider owned XGS-PON compatible one and a new modem. But for your own equipment you have to manage everything and make sure it complies with their published specifications.