TL:DR: it's not possible to remove a primary Gmail email address (*@gmail.*) from a Google account without completely deleting the account associated with it (Google Account allows access to all Google services, whereas a Gmail Account only grants access to email). This does not solve the problem as such email addresses will always be associated with the now closed account meaning, if used to set up another Google account, Google will attempt to re-associated the old email with the old account and not the new.
They never make this easy do they.
There may be many reasons for wanting to change the primary email address associated with a Google Account, the (usually) @gmail address that grants access to all Google Services and/or Gmail, unfortunately it's not possible to change or remove it once registered.
There is a 'work around' of sorts however; if the main Google Account (the one used to sign in and view the services associated with the account) has a GMail account (@gmail.*), the only way to change the address is to completely delete the GMail account. When doing this Google requests a replacement email that will subsequently act as the login email, replacing the old and allowing account access.
For now this appears to be the best solution, and is obviously entirely dependent on wanting to keep, or not, any old Gmail addresses,inbox emails and messages.
