Ye, it's trickier.
Are the values a list of values or is it open text?
Let's take your example, I think that the field now has "Email Preference,; Blog" and you want to remove Email Preference?
You can use the filter Data value changes and add the previous value constraint
Data Value Changed
Attribute : Name of the field
Date of activity : yesterday's date
Previous Value : is Blog
Flow
Change Data Value to Blog
The down side is that you would have to run an individual campaign for each value.
It might be easier to export all updates leads and remove the first value in your speadsheet and then re-import?