- You have an old Gmail (A)
- Create a new Gmail address (B).
- Forward all new mail from the old address (A) to the new one (B) and set to automatically trash the new mail in mail A (it will count in the space only prior to permanent deletion by gmail, which takes some 30 days). Set to never send to SPAM, always to forward.
- Set B to send mail as A (you use mail B, but you may send as mail A).
This is pretty much it.
P.S.: Also see