If I create a new email address will the old address diasppear and be unusable?

It depends on you. If you don't access the old account at all in a 4 month period, Yahoo will automatically deactivate it and erase all content. However, if you check the account periodically, it will remain open as long as you want it to be.
If I create a new email address will the old address diasppear and be unusable?
As long as you don't delete the old address, it will always be usable though most providers will close the accounts if it is not used for some specific number of days.
Reply:No. Most email service providers don't limit the amount of email addresses a person has. They don't even really keep track of the amount of addresses a person has. I myself have about seven email addresses.
Reply:no