Resolution
If Parallels Plesk Panel is registered in Parallels Plesk Expand it is not recommended to upgrade the panel to version 9.x.Parallels Plesk Expand version 2.3 is not fully compatible with Parallels Plesk Panel 9.
The integration is under development now. New version of Parallels Plesk Expand that works with Plesk 9 assured will be available in future.
In general integration of Parallels Plesk Panel 9 and Plesk Expand is possible and everything should work usual way. However it was not tested completely and problems are possible.
One of the problems is usage of Centralized Mail server.
If Parallels Plesk Panel 9 is assigned to Centralized Mail through Parallels Plesk Expand then buttons Mail Accounts and Mail are not hidden in Parallels Plesk Panel. They still lead to local mail server and may cause misconfiguration in mail service.
You may hide buttons Mail Accounts in page presets manually. For details refer to this article: http://kb.parallels.com/en/5792.
However button Mail in left Navigation Panel (Main Menu) under client login cannot be hidden. It still leads to local mail server and may cause misconfiguration.