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.
If anything breaks on the network side, the troubleshooting procedure is "connect the hardware we sent you and see if it works". If it does, it's up to you to fix your side. If that requires new hardware, you're kind of screwed. KPN has the obligation to permit you to run your own hardware and to provide you with the information necessary, but not to keep any kind of backwards compatibility.
(Euro)DOCSIS should be backwards compatible, but things like radio channels and unencrypted video signals have already been replaced by their digital equivalents to add more upstream capacity by Ziggo (the last remaining large Dutch cable company). This broke functionality for a whole bunch of devices, but these changes were announced months in advance so customers had to choose between ending their contract and taking it.
The trouble with dealing with KPN is that KPN is also the company operating the POPs in most places, with many other ISPs leasing their lines. So even if you switch to a different ISP in protest of the XGS-PON switch, you're very likely to still end up with a XGS-PON signal from KPN.
I still believe that the original move, forcing KPN and other network owners to allow competitors on their network, was a better option than digging up the streets twice to get two fiber networks in place.