I've read this too in an article by Mike Miller who is a senior BMW tech. The reasoning is that the new fluid may cause sludge to dislodge where it was previously causing no harm. This may cause the transmission to slip.
In either case, I'm in the same boat as you and am not changing my ATF fluid at this point. Now if I was noticing transmission issues, I'd certainly consider it, but mine still shifts just fine.
2015 BMW 435i GC MSport (ED Sept 29, 2014)
2004 BMW 530i (retired)
2010 Lexus RX350 AWD (hers)