Not RTFT but have read the OP’s comments.
My eldest got to the end of his reception class and when he was moving to year 1, not only did he not have a single friend with him, there was also only about 5 kids from his previous class going into the new one.
I talked to a friend of mine who not only had children in the school, but also helped out in the school (and later went on to become a TA then a teacher). She told me not to worry - she said that in all her years she had never seen a wrong decision over placement in new classes and that they spend a long time working out the best groupings for the following year.
Academically, she was right - he absolutely shone from year 1 onwards and perhaps separating him from his friends who were distracting him was the best plan. Friendship wise, he had friends still - but that friendship group from reception are still going strong now (aged 17) and he’s not part of that at all which is a little sad. He’s never really had many friends now that I look back and think about it - but the ones he does have are GOOD friends, really genuine and the sort he’ll have for life.
Would his friendship group have been different had he stayed with them? I have no way of knowing. But I do know he’s now looking at Russell group universities and if he hadn’t knuckled down from an early age, maybe he wouldn’t have that opportunity now!
Your daughter will be ok. They make new friends much easier at the younger age. The school is privy to a lot more than you are - maybe she sees that the friendships aren’t healthy for your daughter (my youngest was separated from his cousin after reception class because the school saw that she was bossing him around, he’d do whatever she said/asked/told him to do for example - but I didn’t see that out of school) and so they want to give her the chance to make better friends. Or maybe they’ve felt your daughter needs more support academically and do have put her in a different class. Or that her friends need more support. Perhaps your daughter is gifted so they are putting her with a group to push her.
Many schools do lots of combined year group activities, so it may only be when she’s in the physical class room that she won’t see her old friends, but for things like PE, drama, etc they might all work together still!
Please try not to worry.