I think a better example would be the initial release of a book and a later "errata-included" version of the same book.
As I understand it, the CM will use the same import/export feature that we can currently use.
A "re-export" of modified content can be "re-imported" into other realms using the previously exported information, and the new import will update the Topics that were added in the previous import.
It is important to note that Topics are NOT matched by name. They are matched by a hidden internal identifier that is unique to the topic in the realm where it was created. Exporting and Importing preserves this identifier and allows the update mechanism to work.
Example
- Create Topic "A" in Realm 1
- Export Realm 1's content
- Import Realm 1's content into Realm 2
- Topic "A" appears in Realm 2
- Modify Topic "A" in Realm 1
- Re-export Realm 1's content
- Re-import Realm 1's content into Realm 2
- Topic "A" in Realm 2 is updated to match the version from Realm 1
Note: It is also possible to export a changed version of Topic "A" from Realm 2 and import into Realm 1 to have those changes apply there. However, in practice, this could easily become a mess of managing which one has the latest correct version, and so on.
At one point, there was mention of being able to reject changes being applied by an updated import. I don't know what state that is in now.
By comparison, if you make a COPY of Realm 1 to create Realm 2, even though both have Topic "A", the one in Realm 2 will have a new identifier unique to it, and exporting changes from Realm 1 to import into Realm 2 will leave you with two slightly different versions of Topic "A".