Second, you assume that your friends at elementary will remain the same as they grow into teenagers and your relationship will of course remain the same. (isn't this the premise of EVERY angsty coming of age book?)
Here is a curious but not uncommon story. Sophomore year at Bowdoin in Maine, my daughter moved into her social house and introduced herself to the girl next door from Boston. There was a picture on that girl's wall from her tween summer camp in Rhode Island. A group of girls at the beach with their arms around each other. She said, "heh, isn't this you?" pointing to her picture of 12 year olds. "You are the only one in this picture, I didn't know". That is not coincidence, it was only a matter of time.
When the middle school placements come out soon (we hope), there will undoubtedly be tears. You will want to cry also, but don't - you are the grown up. Help them understand that life brings surprises, sometimes wonderful ones wrapped in crappy paper. They may find their BEST friend that they haven't met yet, but they don't have to lose their old ones just because they aren't in class together.