The rules apply.
There is no difference whether it is in negative or possitive
Lets see the rules
Uses of Ser:
Ser is used to talk about permanent or lasting
attributes. If this general rule is too vague for you, think of the
acronym DOCTOR, which stands for Descriptions, Occupations,
Characteristics, Time, Origin, and Relationships. Let's take a look at
each of the above categories individually.
Uses of Estar:
Estar is used to indicate temporary states and
locations. If that general rule doesn’t suffice, there are two
acronyms that you can think of, PLACE and LoCo. PLACE stands for
Position, Location, Action, Condition, and Emotion. LoCo stands for
Locations and Conditions. Let's look at PLACE now
Spanish Dict "ser" vs "estar"
La reunión, sin ser urgente, es necesaria para prevenir problemas.
In this case "ser" is used with time (for urgent).
It is not used for location, action, condition or emotions (rules of estar) so in this case estar can not be used instead of ser.