It is possible to modify non-registration-relevant data of an existing product even after registration.
In order to ensure that the latest version of the product data is used by ETS, increment the "Non-reg-relevant Data Version" of the Application program, Hardware, or Product.
For adding languages, consider using the Merge feature instead.
Updating ETS3 data
It is also possible to provide an updated version of ETS3 data for ETS4 / ETS5 users. Start by Converting the vd file, specifying ETS4 or ETS5 as target. Then edit the data as required and increment "Non-reg-relevant Data Version". Create the updated knxprod by Building the project.
Note on ETS5
Due to the fact that ETS5 copies used product data into the project store, importing a new version of a product will not update data in projects where the old product data has already been used.