It is good practice to replace all four.

It is true that the front to rear difference can cause problems with the transfer case. Also, different tires brands can have different rolling diameters in the same size, so if you do mix them it is safer to do so if they are the same brand and model, as well as being the same nominal size.

BMW's spec for allowable tolerance in rolling diameter is posted here, from memory it is a few revs per mile difference. If you use the same brand and size tire, you are safe to about 50% tread difference in wear as a cutoff. So, if your old tires have 60% tread remaining, you can buy two. If they have 30% remaining, you should buy four.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue