The 9/14 patch notes contained the following note:
Fixed a bug with and re-enabled new spells replacing old spells in the hotbar if the new spell is more powerful.
Apparently, there was more than one bug. There are reports that it is now replacing the existing spells regardless of whether they are more powerful or not. The good news is that they also snuck in a flag to turn it off. Rothgar posted this on the official forums:
We are looking into the issue, it shouldn’t be replacing more powerful spells with weaker ones.
In the meantime, you have the ability to turn it off right now if you place the following line in your INI file.
replace_spells_on_hotbar false
We will be exposing this option to the Options window soon.
So add that line into your eq2.ini file in your game directory (or create it if it doesn’t exist) and you should be good to go.