Both 5.1 and ACS 4.2 ACS appliance support authentication against LDAP and local DB.
ACS 5.1 is the next generation ACS platform and provides a policy based mechanism for defining authorizations; as opposed to the user/group based mechanisms in ACS 4.2.
To migrate the system from ACS 3.3 requires a two stage process:
1) upgrade to ACS 4.2
2) migrate data from ACS 4.2 to ACS 5.1
The second migration process can extract all user/device definitions from the ACS 4.2 to ACS 5.1 and then need to create the appropriate policies that define the user access
The ACS 5.1 DVD set should include all the required software versions to perform this upgrade although I am not familiar with the specifics of upgrade of ACS 3.3 to ACS 4.2. The original 3.3 system could be kept in place and the upgrade/migration be performed on a parallel system.
ACS 5.1 does have capability to import user/device data from a csv file and so if can get the data in this format can avoid all the upgrade/migration related activities
Hope this helps