Alaska did exactly this in 2019. I had purchased discounted QF tickets 9 months before travel expecting to earn 100% of flown miles. Then 5 months later Alaska dropped the earning rate to 25%. I wrote to Alaska twice asking for an adjustment for this (to me) retroactive change. Alaska refused twice.
Honestly should be illegal but FFP can do whatever they want ... sigh.
Should be limited to only making changes applicable to the next year. But thats my fantasy. If they do that I would just move credit somewhere else and stop flying AS as much. I feel like there is good chance the BA bonuses don't make it much longer.