You are correct that only a court approved DNA test is sufficient for changing any details.
I don’t whether you are looking at changing the name on the birth certificate or whether the mother is but here is an article in exactly that subject:
I don’t know whether you have been paying Child Maintenance and whether you’re still with the mother or not you are not and you were paying Maintenance, you may be able to get it refunded
In the interim, as far as the law is concerned you are the child’s father and are responsible for paying Child Maintenance and to have contact with the child until it’s officially decided otherwise.
Depending on the age of the child, you may be able to continue to have contact if you wish.
Can I clarify anything else for you?
I’m happy to answer any specific points arising from this.
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If you still need any point clarifying, I will still reply because the thread does not close.