What are you trying to change Dismemberment to? There's two different values that determine dismemberment status:
Dismemberment: This determines if dismemberment is ON (1) or OFF (0). A value other than 1 or 0 will default it back to 0.
Dismemberthreshold: This determines how much damage needs to be done to a joint to dismember it. 100 is the default. Lower values require less damage to be done to a joint to dismember it, and vice versa for higher values.
I'm guessing that maybe you were trying to change the Dismemberment value and not the dismemberthreshold one?