changes to CM when a child moves in with the paying parent

Hi
Can anyone advise me on my best way through this?

My ex and I have two sons. I have been their primary caretaker for 10 years but now the eldest (17) wants to live with his dad. What will happen to the CM payments? (The youngest 15 will stay with me).

Our Court Order had my ex pay £500pm for the two children (with RPI). There is a clause that if my ex were to go to the CMS and had this amount lowered that the difference would be made up by Spousal Maintenance (Spousal was being paid at 1p a year so it could be varied). Two years ago my ex chose to go to the CMS and was told his payments were £265pm. He elected to pay this and the 235pm in Spousal Maintenance. (I am not sure why. I didn't ask for this arrangement and he did not go to court to vary the SM - he just chose to do it).

And now our eldest is going to go and live with his dad for his last before uni (hopefully). So how do we go about a fair arrangement here? Do we have to go back to solicitors / court / mediation? Also i don't know what to expect or ask for.

My ex earns more than me and lives with a partner ( I don't. Also there are no other children involved). I don't know whether the SM is binding if he didn't go to court to vary it so i may lose that?

Also i assume that the CMS can only override our CO until our eldest is 18? At which point we revert to the CO (which states my ex pays half the CM to the eldest until after his degree and the other half for the upkeep of the other child?)

It seems a bit of a quagmire and i would really appreciate any advice on how and where to get started on sorting it.

Many thanks

CMS could potentially be payable up until the age of 20.

If you are on good terms with your ex it is probably worth asking what has plans are and what he expects from you. The obvious course of action would be for him to inform CMS that eldest son now lives with him and get an assessment of how much you need to pay.

That will leave you paying him for the eldest and him paying you for the youngest.

One of you will then probably need to apply to court to get the global aspect of the court order amended to take into account the new circumstances as it doesn’t cover this scenario.