Navigating a long beta period can be tough for a marketer. At worst, it can lead to you losing the opportunity to shape perception of the product and taking the wind out of your sails. When you launch the "beta," it can be hard to drive excitement given limited availability. Then by the time you "launch," it's no longer news.
Usually the beta period is required for some really strong technical reasons. In those cases, I've seen some smart ways of turning a beta period into strategic part of a GTM strategy, rather than just something to overcome. First, understand what types of users will help the product team achieve the needed learning from the beta. Then, without those parameters, think creatively: Is there a group of users who have long been underserved that you could offer the beta to first? Is there a group of influencers in your userbase who can model how to use the beta effectively for the rest of your community? Is there a way you can capture helpful marketing content from the beta users to inspire everyone else when you expand availability?
When product changes significantly, I always think it's best to be candid in your marketing about what led to the changing. Usually the evolution is because of really smart and sharp user feedback and increased clarity internally of what the product feature is designed to achieve. By explaining the rationale, it can help land your value prop with your users with more precision.