There are two types of rebranding: one is Proactive rebranding and the other is Reactive rebranding. Proactive rebranding is done when a company recognises that there is an opportunity to grow, innovate, tap into new businesses or customers, and to reconnect with its users.
Reactive rebranding is done in a situation when the existing brand has be discontinued or changed. Possible reasons for such a action could be mergers & acquisitions, legal issues, negative publicity such as fraud, aiming to beat the competition, or create your own niche.