I have introduced about 30 fish to my pond over the last month with, as far as I can tell, no adverse affect. This has been due to fish being cheap and some friends of mine wishing to thin thier stock.
I was always led to believe that winter was the best time to move fish! are Koi different to any other fish?
The only fish I have lost found its way into the skimmer

The cold water does not bother me if they are comming from outside and going to outside. A little caution should be exersized if moving from inside to outside(ie, from a shop to a pond).
Sounds like health and safety gone mad bought down to Koi level.