When you change the default of a preference, such as default email level, you can choose whether this should also apply to existing users, but when a user has changed it themselves, their choice is not overwritten.
Example:
The default email level is “only when away”.
User1 changes it to “never”.
User2 does not change it, so it’s still “only when away”.
You set the default email level to “always” and choose to apply the changes historically.
Then new users will have “always” in their preferences and the preference of User2 is changed as well. But User1 will keep their preference.
If you absolutely want to overwrite the decision of the user, you can trick the mechanism by first setting the default to “never” and applying that historically. When you change it to “always” afterwards, the preference of User1 is changed because they now use the default too.