By my calculator, the revolutions per mile on those two tire sizes are 717.0 and 703.7 = a difference of 13.3 rev's per mile. I'm assuming that the rear tires are close to the 703.7 value. What matters is the match from front to back (side to side too, of course) - so if all 4 tires went up to around 717 that would be fine.

That is too big of a difference, according to your owner's manual I expect (it is for my E53, which is different).

The excessive difference can cause problems with the transfer case. With later models than mine (2001), x-drive maybe?, there are other concerns for when the AWD computer thinks slip is occurring when it sees an excessive difference and tries to compensate.

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2001 X5 3.0i, 203k miles, AT, owned since 2014