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 only guarantee is that they'll give you a new provider owned ONT and router during the upgrade. But that's not very useful if you want to keep running your own equipment.