Our daughter has changed schools several times. Social adjustment was a concern, but what we've found is that instead of losing friends, she just seems to collect more of them. So she's still in contact with friends from a couple of old schools (though she's maybe not as close) as well as having friends from her new school.
Now, this is going to depend on your child and their strengths. Shoot, DD went on a shadow day at a local private school and told me "I just made 5 new friends!" In fact, she makes better friends at a place that's more appropriate academically.
Still, I was told point blank by a school psychologist early on that changing schools would be terrible for my daughter's emotional and social health... and my husband (who grew up a military 'brat') just laughed.
