Finance change management often involves restructuring or changes in financial processes. How transparent should leaders be about the potential downsides or challenges associated with these changes? How should organizations manage possible negative morale implications?

Similar to another thought in the thread, focusing on the benefit of the change will assist the teams digesting and understanding the rationale. Being transparent about the risks or downsides will show that you also were thoughtful with the change management.  It is important to map out who is impacted by the change and how each team or individual will be impacted. Some individuals will be OK with an email, while others appreciate an all-hands, and some would prefer a 1 to 1 discussion.  

If you believe the change will be viewed negatively, I would recommend setting up monitoring of morale through surveys or additional 1 to 1 discussions. Listening to concerns in an open manner, but ensuring there are clear expectations that the change is going to happen.  You may find that some concerns are fixable or additional communications are needed. 

Change is always difficult but its necessary. In my experience, being frank about downsides and challenges is important but more more necessary to focus on the benefits of the change (I am assuming here that the change is more beneficial to the company than the downsides of the change). Be frank but limit focus on the downsides.