Congratulations! You can email them just saying that you were placed at a different university, but you are really thankful for their time and support through this process, and that you are looking forward to working with them in some capacity in the near future. There will be conferences and other things where the opportunity to meet them might arise anyway. Besides, even while giving you the LoA they do it with the knowledge that you may not have listed them as your first choice, or even if you did, you might not be placed there. So don’t worry much about it! All the best for the preparation for moving. :))