Q: apple ID

I have two apple IDs and only using one. (Let my call it ID no1 and no2)

Because Apple ID cannot be erased, I have already changed the Apple ID e-mail address of ID no1.

And would like to change my e-mail address of Apple ID no2 to what I was using for ID no1.

(ID no1 was linked to frequently used e-mail address, but ID no2 is linked to all my apple devices)

But when I tried to change the e-mail address of ID no2, they say this e-mail is already used as Apple ID

even though I have changed the e-mail address of ID no1. Seems like my change is not reflected

or if you once register the e-mail address you cannot make a change like this?

Thanks in advance.

Yukinoning