How dangerous can it be? I am going to research.
This article mostly agrees, except if it is less than 30 percent worn, you can just replace one. https://www.consumerreports.org/cars/tires/should-you-always-replace-car-tires-in-pairs-a1061792056/
Why put the new tires on the rear axle though? I thought the front was more important for braking and front wheel drive, which i have.