If you upgrade now, your sandbox will be built on a non-preview instance, and roll back to Summer '18.
Here's how it works:
Sandboxes can be on "preview" or "non-preview" instances. Preview instances automatically switch to the new version at the beginning of the preview window, which, for the Winter '19 release, runs September 7th through October 12th.
Regardless of whether your sandbox is on a preview or non-preview instance, if you refresh your sandbox before the beginning of the window, it will be moved to a preview instance, and be one version ahead of your production org.
Likewise -- regardless of whether you're on preview or non-preview -- if you refresh your sandbox inside the preview window, it will be moved to a non-preview instance, and be on the same version as your production org.
Being on a "preview instance" means your sandbox will be automatically switched over earlier than your production org ...but that's all. As far as refreshing your sandbox goes, all that matters is whether you're inside or outside the preview window.