We have just moved to a new area for the start of year 10 with DD and year 7 DS. It has been really tough for DD - really bad organisation from school despite having good policies, teenage girls, closed cliques, exam pressures, different syllabuses covered and needing to cover all of year 9 Science in a term with me at home, etc, and considerably easier for DS - boys have different kinds of friendships, everyone else was moving too - though still not at all easy. On the other hand we had no other options, and sometimes you just have to do it for a bigger issue. They will cope eventually, though I have needed to give LOTS of support.
I think it matters to help manage their expectations in advance. It will be tricky for quite a while. They are grieving for what they had. Looking at our two I think for DD it could take up to two years before she feels settled/at home. Currently has a friendship group to go to at lunchtime but doesn't feel she can be herself with them (3 months in). Whereas DS is being invited to parties and happy to go to them.
We moved to a very settled fairly insular community because this is where we could get them into school. In retrospect it would have been better to move somewhere with more coming and going, practice in incorporating newbies, and less of a sense you are odd if you come from outside.
I have needed to promise them that if they feel it is really dire still after a year we will look at other options - ie other schools in the area - but have reminded them this would mean they would be new AGAIN- , so they don't feel completely closed in with no choice. We do marks out of 10 on our days regularly, so I can get a sense of quite how dire it is, - sometimes it has got to 6/10 for DD, but regularly tears and 3/10 still. On the other hand I do think, slowly, things are getting a bit better or at least familiar.
People advised me to support them to go to scouts etc out of school to make links there which can come into school. What this underestimates is that if you feel actively miserable and isolated at school you can't bear to run the risk of this happening again in your freetime. Have managed to get DD to drama club outside the catchment area which she is enjoying, - but she is clear she doesn't want to be seen where people from school are, because she feels so exposed with them. So its like keeping a little candle burning in the darkness that there are people who like you - but not actively helpful to build bridges.
Sorry not to be more positive. In general for the family this had to be done, and in the long term I think it will have upsides. But this initial year is pretty grim, much like the transition to sleepless life with a newborn and a toddler. Good luck.