Well, a week since the GDS change, and both CX216/219 and CX357/358 remain bookable.
While its clearly a flight number change, I don't buy the excuse that it's to do with safety and conflicting with CX257/258. I would hope the highly trained and large aptitude ATCO's can differentiate between a 2 and a 3, there are lots and lots of similar flight numbers around the world that survive quite happily. As a MAN example, we have EK21/22 and EY21/22 which no doubt cross each other somewhere over Europe on a daily basis.
It also doesn't explain why they went for CX216/219 rather than CX216/217 or CX218/219 to correspond. To me it does seem they have left a gap to add in a 2nd daily at some point. As said, not a full 14 weekly to begin with, but either 9 or 10 weekly, possibly Mon/Sat 2 daily as they tend to be the stronger booking days.