Because you can't create aliases in standard Gmail accounts, creating a new account is the only way. You can leave your old messages in the old account and have both running in Mac Mail (or other email client software).
For a true changeover, I think you'd want, as has been said, to create the new one and set up forwarding at the original address (Gmail website) to the new address. Then you could temporarily have both addresses in Mail and copy the messages from folders on the old account to corresponding folders in the new account. Once the copying is done, you could remove the old account from Mail.
Contacts and Calendars would have to be dealt with separately.