welcome Sandra, quite the decision you have to make. Although I don't live as far north, am at the Pennsylvania/Maryland border. I would personally try your idea before tearing down the pool and digging and installing a new pond. I would consider purchasing an outdoor air pump and bubbler to keep an air hole and a modest pump and filter and just a few goldfish and see how it does come spring. If all goes well you can upgrade your pump and filter and maybe even get more fish to what will be more suitable for the pool pond.. at the least you will have an second pump and filter to help... oh i remember reading somewhere once that people up north have used bales of hay/straw around perimeter of pool to help insulate and even use clear heavy gauge plastic to create a slight warming effect...just remember to leave space between water and plastic for air movement and so plastic doesn't freeze to water...maybe even using one of those inflatables to keep the plastic aloft... not sure if it will work but wish you luck with your decision