As a reminder, The War Within includes several API changes which will require addons to be updated before they work properly in the pre-patch, making now a very good time to back up your addon settings and WeakAuras!
Addon versions can also be checked on their Curseforge or other repository webpages.
While popular addons like ElvUI, Deadly Boss Mods, and WeakAuras will already have already been updated regularly throughout the beta, and may be ready to go by the time servers come up, copying all of your addon settings (and particularly your WeakAuras) from your \World of Warcraft\_retail_\WTF\Account folder to any other backup location will help ensure nothing gets lost or irreparably broken in the update.
Players using addon managers can also easily check the Game Version to see if an addon has been updated to v11.0.
This doesn't necessarily mean every addon will break, some simpler addons continue to function even after several expansions without changes, but players should be prepared for several rounds of updates over the coming week.