when i try to change my email address it says it is used by another apple id

some time i don t have acces to my old email address therefore i changed my email address to a new one . when i tap on my name on my iphone it requires my ols password which i cannot change after chatting with the yahoo people.

when i try to change the old email address it tells me the new on which is also listed in my id is used be another apple id

do i have to delete bothe and then insert the new one again ?

thank you

i am actually refering to my wife s iphone an iphone SE