Is it possible to convert existing users from SQL Server authentication to Windows (AD) authentication? (SQL Server 2008 R2) We have just undergone a bare-metal restore of an SBS 2003 server running our SQL database and are in the final stages of configuration before going live again.

Previously, we had 20 users accessing this database via SQL authentication. This wasn't ideal, as users had multiple sets of credentials to remember-but the person who set it up decided to do it this way. (Any ideas why someone would choose this over AD authentication?)

I am wondering if there is any way to move from using SQL user accounts to AD credentials? If so, what would be the best practice method for achieving this with an existing database?

Any advice would be much appreciated!
The security concept of SQL Servers differentiate between login and user. A login is at the SQL Server instance level and can be mapped to different users for different databases, as long as the database user is a SQL user and not a Windows user.

You could create the Windows login and the user (both having the same name) followed by script that copies the permissions from the SQL user to the Windows user. (Search for "sql server clone user permissions").