Not sure why do you want to replace the code, however, I see two possible reasons:

1. Google Analytics time to time change the code to offer new features. In such case, you can actualize the code on your website and you will see all historical data in your Google Analytics account.

2. If you want to use a new Google account for some reason, than you can use your old account and set up permissions so you can see the data using the new account - without the need to update your code.

If you set permissions to your old account in your new one, you can see the old data,
in any case, why would you change that account?