You've got to warn players before they log in, after you do an update. Please, answer these questions:
- Is there any new GUI element? Will it work with skins other than the client base?
- Do they need to update their art folder?
- Will there be maintenance and retouching updates for some time?
Changelogs are always appreciated. Not trying to tell you how to do your job, but you could have a changelog ready for when the update's done.