I lost a similar amount when I moved to NZ. BA wouldn't recognise my NZ address as all frequent flyers down here were the domain of QF under their joint venture. I couldn't use a reliable UK address as no parents left. So the account perished. I had to switch to QF (and Air NZ for my Star earnings with BMI dead) and plodded through the ranks to lifetime gold but about ten years of platinum. Now the JBA is over BA want me back (their footprint down this part of the world is very skinny with the SYD operation).
I don't think that FFP clause in the JBA was particularly smart if the JBA doesn't last forever.