I think the agent you spoke to was probably a little confused. If you cancel you obviously don't lose all the points safely in your DC account. What you do lose is any miles that you have earned via credit card spend that have not yet been transfered to DC. e.g. If you spent £1000 last week, the 1500 miles you are due would not be transfered to DC if you cancel the card before your next statement date.

If you haven't spent anything on the card since your last statement then you can safely cancel with no loss of miles.