SEP version upgrade: is it possible to use existing content/definitions?

I've to upgrade a client, let me say, from version 12.1 RU1 to 12.1 RU2.

The client already has, onboard, latest definitions.

If I use a package with only basic content for the installation, will the client use the existing definitions or it will download a new full package?
I'd like to know if with a version change we can only deploy the "light" package instead of the heavy one (if target clients already have definitions).

I hope this could be possible since the definitions sets are the same for different versions.