In the Mockingjay book, Effie Trinket has only a tiny bit part at the end, where she appears as a rather unconvincing 'rebel' to help Katniss with her costume and preparation for the
assassination of the president (but which one?)
In the Mockingjay films, she has a much more major role, replacing the book's "Fulvia" as Katniss's District 13 stylist throughout.
Has any official reason ever been given for this change?
An obvious reason is that the filmmakers didn't want to underuse an actress of Elizabeth Banks's profile and talent (although they've done exactly that with Gwendoline Christie in Mockingjay Part 2), but have they ever said explicitly that this is the reason? There are various other possibilities, such as that they didn't want to introduce yet another new face in District 13, or for the rapport between Effie and Haymitch