Separating , jointly assessed.

So basically will be separating and we are jointly assessed with me having all the transferable tax credits.
So we want to transfer the credits back and I will be applying for the SPCCC credit.

So how do we go about this?
I see that SPCCC credit can’t be claimed if you are jointly assessed.

So ideally I would like to have this sorted for the 1st of January so I can claim the credit.

We are amicable and won’t be getting a legal separation, will be doing mediation and getting that signed off and then hopefully the divorce changes will have been enacted and can go straight down that route.

So can we change the joint assessed status anytime and then claim the credit straight away?
Or if your jointly assessed in part of the year does that carry on for the whole year?

If we change the status before the end of this year will that have tax implications for the year gone?

If it does and we leave it till the start of next year does that impact the ability to claim SPCCC for the year?

So basically I am asking how do we go about this

Thanks in advance.