Looking at the handback rules it's mentions ADSL but not FTTC to FTTC migrations
Handback Rules:
This deals with performance issues relating to a service that has been in service for less than 1 year and cannot be adequately fixed by Openreach.
If the line has been in service for 90 days or less, the line can be ceased (if the line was a new provision), or reverted back to ADSL (if the line was migrated from ADSL), at no cost to the customer. If the line is older than 90 days but less than 1 year, the line can be ceased (if the line was a new provision), or reverted back to ADSL, (if the line was migrated from ADSL), the only charges that will be raised will be the line rental for the period that the line was active.
The service must have been investigated by Openreach for under-performance since its Service Activation date (i.e. the line will not synchronise at a speed above the 10th Percentile point of sale predicted rate, or the line rate has dropped by more than 25% over a 14 day continuous period).