I don't know that it's a great plan to do a blogpost and in-app notification as the first round of reminders and email on the day of the change. Both the blogpost and in-app notification requires you to explicitly go on GitLab and see there's a problem. Maybe there's a reason to avoid it, but emailing from the get-go seems like it is the right move for transparency and not rug-pulling.
EDIT: clarified antecedent