If I move to LDAP authentication and the user name remains the same is there any impact to existing data with their projects?
No, there is no impact.
Will the new LDAP authenticated accounts still be shown as the owner of items they were before the switch assuming the user name doesn't change?
Yes, the ownership will remain the same.
Can LDAP /Active Directory groups be used to automate the user account creation process?
There is an API for Site Administration (a COM-based API) which provides access for creating/modifying/deleting user accounts as the ALM Site Level. If you have resources to write a program to interface to that API and to interface to LDAP, then you can automate the user creation process.
Adding users to ALM Projects and granting them permissions in those projects is a secondary task. You would need to interface to the Open Test Architecture API (another COM-based API) to complete those tasks.
Documentation for both APIs comes with the product.