Hi Smart Monkey and welcome to FT!
As far as I'm aware it's consistent whim rather than policy. I have never heard on these boards of someone losing miles in this way. I personally have had points credited following a phone call.
If you're nervous about it (which with that number of miles involved is fair enough) then email DC in advance and get them to say what they will do. If they play hardball (which I very much doubt, particularly if you tell them you're thinking abou joining DC from BA for eg...) then consider a quick mileage run.