Essential question for T&E when modifying a system:
Does it fit?
Can it be switched on?
Does it burst into flames (or some other adverse occurrence)?
Does it work at all?
Can it be used by an average service operator?
If sufficient evidence exists to answer the above then T&E is minimal. However, a UK navigation system we were ordered against all technical advice to fit to an aircraft already fully cleared by a foreign T&E:
Did not fit.
When made to fit - Did not switch on.
When made able to be switched on - Did overheat.
When the power supplies were corrected - Did not work (wrong data bus spec)
After all these were corrected and we had flown 20 times the original estimated T&E time it was capable of use by an average service operator!