Pond Boss Forums POND CONSTRUCTION Renovating a Pond or Lake? Should I drain the water from the old to the new?

Short answer, I wouldn't.

With unwanted fish in the old pond, unless you use a fine micron filter bag to keep them out of the new pond, I'd just let the water go and refill the new pond with clean water.

You could transfer nutrients that you cannot catch in the bag, and if you have any unwanted plants/algae in the old pond it could end up in the new pond via the water transfer.