This has more-or-less already been said but:
a. Is it as easy to 'just' change your secondary school choice at this stage? Because where I live, there was hot competition for places at practically every secondary school (basically, all except the one in special measures) so moving schools at this point wouldn't really be a viable option.
b. EVERY 11 yr old in the country is nervous about starting secondary. Two of the biggest anxieties are geography ("how will I know what class room to go to?") and friendships.
c. There will be some that struggle, but the majority of children WILL make friends in their first days/weeks/term.
d. Even in a secondary that a lot of kids from primary go to, they'll be all mixed up and won't see that much of each other, esp in class.
I'd recommend bolstering your child's confidence and doing lots of reassurance and then seeing how things shake down in September. If you still want to consider a move then, then go for it.
There WILL be children who don't turn up for their allocated places (moving or getting into private school are common reasons why round here, and I expect that's a national picture). If the parents don't cancel the place, the school is required to keep the place for that child for 20 school days, so there WILL be some movement towards the end of September. Therefore, there will be an opportunity for movement in September if you think that's necessary.
Again - to echo PPs - why did you choose the school you chose?