Your existing maintenance plan is unchanged- you should be receiving an email with 2019 upgrade instructions shortly (if you haven’t already). If you want to take a look at SketchUp 2019 right away, you can download the SketchUp 2019 trial, which will give you access to everything for 30 days while you’re waiting for your instructions.
The (new) SketchUp Pro subscription includes some more products and services in addition to the SketchUp Pro client application. SketchUp Studio also includes an annual subscription to Sefaira, if that’s of interest in your business.
You will be able to choose between continuing your “classic” SketchUp Pro licensing (with its annual support and maintenance fee) or switching over to the new annual subscription. There’s no need to make that decision now; both licensing models are supported.