Advice on changing key worker

[SienaSun]

I am thinking about asking nursery to change my son's key-worker. It's a smallish nursery and he's in the pre-school year group. I love the nursery in general so definitely don't want to move him.
How do nurseries allocate keyworkers/groups? Is it by ability, age, days that they attend or just random?? They plan for each child individually so would it matter that much if he switched groups, even if they are at different stages? Hoping someone who works in nurseries can advise.

I allocate key children based on a number of things, mostly attendance
I also look at other things such as temperament of the child and the member of staff. I don't want to put an anxious child with a very loud person. My staff have different skillks and some children may her more help settling in and some staff are more skilled at settling than others
I also look at group dynamics and make sure the key person doesn't have too many difficult children or boys or summer borns or send children, as they all need extra support
Sometimes though, it might be that the child makes a beeline for a particular member of staff and the child decides
Why do you want to change?