I wouldn't go that far. Tires are worn out when they have 2/32" left, so 50% worn is 6/32". But if they are the original tires at 47,000 miles and it was my vehicle I would replace all of them. That is a downside of an awd vehicle.

You have just as much of a problem front/rear as you do side to side. Side to side is hard on the diff, but front/rear differences are hard on the transfer case, and that is where issues have shown up with X3s with mismatched tires.

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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