The Assign Switch will normally default to Off when a new program is recalled, unless you have set the CONTROLLER RESET parameter to HOLD.

Press [UTILITY]
Touch "Settings" > "MIDI I/O"
Set the "Controller Reset" = Reset

This parameter is responsible for how your physical controller behave when a new program number is recalled. Here is a list of what is Reset and how.

Resets the values set for the following controllers.
PITCH BEND 0 - (center)
CHANNEL AFTER TOUCH - 0 (minimum)
MODULATION - 0 (minimum)
EXPRESSION - 127 (maximum)
BREATH CONTROLLER - 127 (maximum)
FOOT CONTROLLER 1 - 127 (maximum)
FOOT CONTROLLER 2 - 127 (maximum)
FOOT SWITCH - 0 (off)
RIBBON CONTROLLER - 0 (center)*
ASSIGNABLE FUNCTION SW 1 - 0 (off)
ASSIGNABLE FUNCTION SW 2 - 0 (off)
MOTION SEQ HOLD - 0 (off)
SUSTAIN SWITCH - 0 (off)
SOSTENUTO SWITCH - 0 (off)
MOTION SEQ LANE 1 - 0 (minimum when Polarity is set to Unipolar and center when Polarity is set to Bipolar)
MOTION SEQ LANE 2 - 0 (minimum when Polarity is set to Unipolar and center when Polarity is set to Bipolar)
MOTION SEQ LANE 3 - 0 (minimum when Polarity is set to Unipolar and center when Polarity is set to Bipolar)
MOTION SEQ LANE 4 - 0 (minimum when Polarity is set to Unipolar and center when Polarity is set to Bipolar)

* Ribbon can be set individually per program.

Doesn’t reset the following data:
PROGRAM CHANGE, BANK SELECT MSB/LSB, VOLUME, PAN, HARMONIC CONTENT, SUSTAIN LEVEL, RELEASE TIME, ATTACK TIME, DECAY TIME, BRIGHTNESS, EFFECTS 1 DEPTH, EFFECTS 4 DEPTH, PORTAMENTO SWITCH, PITCH BEND SENSITIVITY, FINE TUNING, COARSE TUNING, ASSIGNABLE KNOB 1-8

Ah so it's a global setting. I have it set to Hold as I'm using an FC7 for expression and I don't want a jump in level when I change performances. I guess I could filter out ASW 1 from the next performance if it affected any of the parts which would give me time to turn it off when I had a free hand.