Basically two J long hauls or 3 W will get free selection from time of booking for up to two years, and 1 J and 2 W will get it from 7 days out (plus a few more flights somewhere to pick up the residual TPs and BA marketed flights). Everyone gets a free choice eventually too.
So loyalty is rewarded very quickly indeed. It's probably the key BAEC selling point, so BA are hardly likely to reverse it. No-one with status is going to want it changed.