Got you so far.
In that case any CP can get Openreach to do a "return to donor" = "Conversion of Local Loop Unbundling (LLU) Metallic Path Facility (MPF) to a single Wholesale Access line" and pass on the Openreach charge or not.
Currently then BT Retail don't pass on the charge.
It's just a case of your chosen provider knowing what to do and not just arranging a new line if you move from MPF back to BTw.
So far so good?