As others have alluded, it's all about rolling radius. I've always gone with 4/32nd as a rule of thumb. So if the most tread you have on one tire is 5/32, and figuring a new a/s tire is 11-10/32, you would be out of that 4/32nd recommendation. See the article below as other manufacturers have even stricter recommendations. Regardless, 29k is a lot of miles on any BMW.
https://blog.tirerack.com/blog/make-...on-an-awd-car-