Possible to “swap” OAuth2 Gmail API scopes?

Question

We have an app in the Google Apps Marketplace that currently uses the Email Settings API:

https://apps-apis.google.com/a/feeds/emailsettings/2.0/

We use this to get a user's email signature.

This is being discontinued later this year, so we now want to move to the Gmail API and use this scope:

https://www.googleapis.com/auth/gmail.settings.basic

We already have the Gmail "compose" scope, however this doesn't allow us to retrieve the signature.

We have over 250 businesses using our software and changing the scopes within the Marketplace SDK will trigger an enormous amount of stress / effort / confusion if end-customers have to re-authorise our App. We'd prefer not to inconvenience the end-customer when we're essentially asking for capabilities that are lesser than what we already had through the old Email Settings API.

Is there any way we can just "swap" and downgrade scopes, or add the new scope without customer intervention? Google have done similar things in the past with deprecated APIs.

EDIT: in case it matters, we're accessing the Gmail API using a service account so we can access the details of any user like we could with the Email Settings API.

(If the answer is "no", I'm hoping an official Google person will see this and take this idea away for consideration. Alternatively, allow us to retrieve signature settings with the "compose" scope since appending a signature to an email you're sending is a genuine use-case.)