Linking current account with Google without losing ratings.

You can go to 
https://www.imdb.com/registration/accountsettings
and change the email address associated with your account.

Notice that a link to Help is atop nearly every IMDb page, and searching for "change email" would have led you to this answer.

Hi Jimmy,

To add to what bderoes said above, in order to ensure that you are using "Sign in with Google" to access your current IMDb account, you should first be sure that your IMDb account is registered to the same email address you use to sign in with Google.

Once the email addresses match, you'll be able to simply select "Sign in with Google", which will link your Google account with your IMDb account for future sign-in automatically.

Let us know if we can offer any further help.