To see what entities are used select File | List Entities. This will show a list of all the entities. A single entity can have data files for several years and will still only be one entity. The entity name is the name on top of the reports.
So unless these are new entities, there is not problem using the same 5 entities as last year.
Note: Do not put the year in the entity name since it is already on the report.
When an set of data files is deleted using File | Delete such as old years, there is an prompt to remove the entity from the entity list. If the entity name is removed then when the data files for the current year is accessed it will be prompted to add the entity name back to the entity list.
Hope this helps to answer your question.