If so, how do you announce it in a newsletter without it looking like spam to them?
100%. Many companies "Pivot" and when they do, they let their current users know that the service is shutting down - but that they're now moving onto something new (or launching a new company).
Flowtown (my last company) did this 3 times. We even leveraged users to test the new product before we made it public.
Just be respectful of peoples time/email and only send to them if they opted in to receive company updates.