First of all, sometimes product launches are about existing customers. Maybe you're launching something meant to dramatically improve their experience, or help them expand their engagement with the product. In those cases, they are your primary audience and all of your comms should be shaped to inform them and motivate them to take the action that you want them to take.
However, let's say that your launch is about attracting new users. You still want to engage your existing users as evangelists, influencers, storytellers, and examples. Nothing is more powerful than someone who already uses your product telling new people how it's transformed their lives for the better. You want to make it very easy for them to do this and build them a platform for their voice to be heard. Supply them with all the information they need to share your news to their followers on social media, showcase how they've been succeeding with your product in customer stories that you post to your website, YouTube channel, etc. Make sure your launch is embroidered with all kinds of examples of existing users' experiences. What they've done with your product. The value they've realized. Your happiest existing customers are going to want to support your success. Make them feel even more appreciated by aligning your interests with theirs, cross-promoting their brands or products, etc. at the same time as you show your audience how your product has helped them.