Thank you for your answers. However, I don't know if I did explain myself well enough.
In my iPhone when I go to Settings, tap my name and tap Sign In & Security I am not dealing with the apple id that is my old email address and I will loose in some months. In fact, here I have no problem. The apple id here is the one @me.com which will be available "forever".
My old email address/apple id is the one that appears in Subscriptions and in Media & Purchases. This apple id is the one that I will loose.
So the steps:
3. Next to Email & Phone Numbers, tap Edit.
4. Tap the Remove button next to the email address, then tap Delete.
5. Tap Choose Another Email Address and enter your device passcode if asked.
6. Enter the new email address, then tap Continue.
will not apply to Subscriptions and Media & Purchases.
However, if I click in Media & Purchases I can Sign Out this old apple id/email address.
If I set it up again I can choose one new email address I've previously add to the old one in anither screen
"Email & Phone Numbers
2 email addresses
These email addresses and phone numbers can be used to sign in. They can also be used to reach you with iMessage, FaceTime, Game Center and more.
2 email addresses"
Curiously, when I sign in in Media & Purchases with this new email address the password has to be the same I use when I sign in with the old email .
iPhone will sign in but the email address I'll see in Subscriptions and Media & Purchase is always the old email address/apple id. Not this new one I've created today and used to sign in..
Am I doing this correctly? Does this will work?
Cheers
Jorge