@charliekrad @Mustrum
There isn't an established process for this unfortunately as it requires a significant amount of work and theoretically could result in some prolonged downtime for the customer, and possibly other customers, even if was feasible.
It would require a swap out of both the ONT as well as switching the OLT/headend on the network/exchange side. The Fibre is passive, so it's the parts either side that require change.
Part of the early Full Fibre deployments up to 2018 or so were these ECI ONTs connected to OLTs from a different vendor (likely also ECI I would assume) to what is in use now for the newer deployments (typically now Nokia or Adtran). There was also some Huawei kit at the time, but this issue appears to be specific to ECI.
Swapping the ONT alone to a different model doesn't solve the issue as it wouldn't be designed to operate with the OLT, even if it could theoretically handle the speed. Also vice versa, the interoperability just isn't there across vendors.
This has been something talked about for years unfortunately, but it's likely a case of Openreach wanting to prioritise new build to those still waiting on any type of Full Fibre before retreading areas that already have access up to 330/50.
A post here on the BT Community from a couple of years ago gives a bit of an insight into the issues: https://community.bt.com/t5/Archive-Staging/Unable-to-order-Fibre-900-ECI-issue-FTTP/td-p/2133666
Probably a lot of information that some will already know on here, but thought it'd better to say it all in one place.
Potentially in the future it's a process that becomes somewhat easier to establish when Openreach look to roll out XGS-PON on the network, but unlikely anything will change for a while.
