BTW: in the EU there is movement towards mandating ISPs allow BYOD, including fibre ONTs.
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Some years ago there was only unofficial documentation even on the parts behind the ONT, like which VLAN carries internet and which one is IPTV etc. Now it's all officially documented and you can run your own modem, router and firewall if you want.
I've left their ONT in place and plugged it directly into a Linux box that does the rest. Gives me more flexibility on things like IPv6 and easier to host local services without port forwarding through their modem.
The provider can transparently run GPON and XGS-PON simultaniously because they run on different wavelengths. However unless the provider can tell all existing GPON customers to replace their infrastructure they cannot stop providing GPON. GPON -> XGS-PON is not an upgrade, it's double the infrastructure where the splitter is.
So my question is quite specifically if there is a contractual way for KPN to turn off GPON and force customers to migrate, or if they are required to service both until the last GPON customer goes away on a splitter.
This has been an issue with DOCSIS for in many places of the world where we are already running out of available frequency spectrum.
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.