Strange, tyre change cant affect the physical tracking (eg trackrods etc.) of the car. All it can be is that the tyres have differing rolling characteristics.
It's an odd one.The first time, the rear tyres were badly worn due to the geometry being out. It may have been that the previous owner had the tracking corrected to straighten the steering wheel. When the new rear tyres went on, the 'square' rear tyres would have changed the rear geometry, therefore causing the car to crab.
This time, there was no uneven wear, but coincidentally, fitting new tyres on the rear caused the same symptoms as before.
What about swapping front tyres to rear?
I wanted the new tyres on the river as the ones now on the front are no longer available. The older tyres will wear more quickly on the front, so can be replaced with ones that match the rear much sooner.