KPN and other Dutch ISPs don't really care about custom customer hardware, on a practical level and on a contractual level. The Dutch standard is that you use the rented hardware your ISP provides, unless you want something special, then you get specs and settings and you're on your own. Even if you use your own hardware, you often still get a modem delivered to your doorstep.
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(Euro)DOCSIS should be backwards compatible, but things like radio channels and unencrypted video signals have already been replaced by their digital equivalents to add more upstream capacity by Ziggo (the last remaining large Dutch cable company).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.
You're almost certain to end up with the exact same line just a different provider on it. Very few areas have multiple fiber networks, although it's getting more common.
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.
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(Euro)DOCSIS should be backwards compatible, but things like radio channels and unencrypted video signals have already been replaced by their digital equivalents to add more upstream capacity by Ziggo (the last remaining large Dutch cable company).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.