First off, I highly suggest you read the LDAP integration guide as well as the system guide that refers to LDAP. It will clear a lot of your concerns.
To answer you questions now.
Once you integrate with LDAP, all the local end users (not applications users) will cease to exist.
So you must make sure any account/end user created in UDS directory of call manager is also in LDAP.
If the alias (user id) in call manager is not exactly the same as your LDAP, then anything assigned to that user will not be reassigned to the LDAP account. If they match then you have no worries.
Anything that does not match will need to be redone manually.
Also any uccx specific accounts will need to be recreated in LDAP unless they are application users.
Finally, I urge you to clean up your LDAP before you integrate as at that point call manager relies on LDAP for ALL user information, including their DN. If it is not correct, user directory will be wrong and also your CCX agents will not be able to login.
In version 9.X and later you can have local users in call manager as well as integrate will LDAP but at the version you are at, that feature is not available.
Unless you are 100% sure you know what you are getting into, be very careful as you can break your call center if done wrong. It is simple to do but with quite a few gotchas, so you might want to bring in a consultant to help you get it going or at least prep it.
Let us know if you have additional questions.