In JIRA 4.2, delegated LDAP would fall back to internal authentication if a user was not present in the directory. In JIRA 4.3, this is not the case.
Please note that the upgrade task will be fixed in v4.3.1 so that we put the users with LDAP accounts into the "Internal Directory with LDAP authentication" and we put the local users into a second "Internal" user directory.
We recommend that anyone with mixed users should NOT upgrade to v4.3(.0), and should wait until v4.3.1 instead.
(If you up to 4.3 and then to 4.3.1, you will not get the fix - the fix only happens when the upgrade tasks run, and in this case it is too late)