They do not expect the upgrade to have any effect on the realms in question, and they say that they will be working to make sure everything goes smoothly. Of course, the unspoken follow-up here is that there just might be some issues. If you play on any of the realms listed after the break, you might want keep in mind that you may encounter some choppy waters tomorrow, and possibly a full shutdown.
The list of possibly affected realms is as follows:
Sisters of Elune
The Forgotten Coast