Depends what you mean by 'devalue'. They (together with other airlines) have a track record of devaluing miles, though the last time BA did this sort of thing a few years back they called it an 'enhancement'. An enhancement to them, a devaluation to their customers.
Do non-redeemed frequent flier miles sit on their balance sheet as a liability? Their balance sheet is truly dreadful right now, probably technically insolvent if you properly value their pension fund liabilities, from an accounting point of view it might make sense to devalue the miles.