Best way to prepare children for a change of school

[Misnomer]

Just a bit of background: We didn't get our first choice school for my youngest child and have been, instead, offered a place at the local catchment school. Our two older children are at the first choice school. We have no hope of getting my youngest a place since we are quite a way out of catchment and sibling priority only applies in catchment here. It's all a long story of how we ended up in this situation but we have visited the catchment school and it seems lovely so we are attempting to move to two older children over to it, especially as the logistics of two school in different directions is very difficult. There is a place for my eldest and the second eldest will be first in the waiting list for the new school (we're very close to the school so hopefully that will work out).

So, my question is how to prepare the two older children for the move. I know that children are resilient etc (some more than others) but there must be better and worse ways of handling it.

My eldest is the least robust. He's lovely but a deep thinker/worrier and has had some issues before around anxiety (was under CAMHs for a bit), which is why we haven't moved him before. This school year, however, he has felt quite unsettled in his friendship groups as the school mix the year groups up each year and he's ended up with none of his original friendship group. Not living locally to the school hasn't helped, I think, so long term the situation will be better with the new school being a stones throw away.

My second eldest is a very secure child, albeit stubborn as hell! He has a small but strong friendship group at the current school and will be very upset at moving away from them. We will be able to meet up still but I'm not sure how much that will appease him in the meantime (we also have no idea when his place will come up so without a definite date it might be harder to prepare him).

Sorry for the essay but I thought it was better to get the info out in the first post!