If my employee has a new manager, what happens to their existing performance review?

All of the employee's performance review data remains the same. All of the previous reviewer or manager's contributions and signatures on performance reviews will remain.
A new manager will gain access to reviews, and if there is an open review cycle, the new manager will see the review underneath the Team tab for the cycle.
However, if the new manager is not listed as the main reviewer then they will only be able to contribute to the review but not share or sign.
The reviewer assigned when the cycle was created will remain unless this is changed from the cycle overview.

You can easily set the new manager as the main reviewer for an open review cycle from the cycle overview.

First, hover over the reviewer name and click the pencil icon.
A menu will pop-up where you can click the trash icon to remove the current reviewer.
From there you'll start typing the new reviewer's name and select. Make sure to save to complete the change.

We have recently implemented an easy way to reset the reviewer to the reviewee's current manager for all employees in the cycle at once.

You'll select the "Admin" drop-down then select "Reset Reviewers" to bring up the selection menu.

From there you can choose to reset to the reviewee's default manager, secondary reviewer, or both.

Note: this is a global cycle change and any custom reviewers will need to be manually updated after you've used this administrative action.

Watch the below gif to see the changes in action!