Companies from Act newer version database got merged in Groups in Act earlier version as Act earlier version does not have Companies
Secondary Contacts from Act newer version get converted as "Primary Contact - Secondary Contact" as Act earlier version does not have a Secondary Contact
In Act newer version there are History and Note tabs. In Act earlier version these tabs are together (Notes/History).
As you mention recurring activities in act 9,8,7,6 to an earlier version are set to recur to 2099.