There's no single cutoff date for the nickel hardware because different lines and even different colors had it available at different times.
The Soho bags had it available on just a few colors in Spring & Summer 1997. On almost all other styles it was made available some time in 1998 or 1999.
This is what I have in my files, you may want to copy it and we can correct it if something new turns up:
I've never seen nickle hardware used on a bag from as early as 1996. The earliest I can find nickle hardware offered as an alternate to brass is sometime in early to mid 1998 EXCEPT ON SPRING/ & SUMMER 1997 SOHO BAGS, SEE 1997 SPRING CATALOG, and then only on a few colors like black, gray and lavender in Spring, and a few more in the Summer catalog.
It was available about a year earlier on briefcases & available with Toffee, Lavender & Pistachio on a few bag and daypack styles in early Spring 98 & with a few new 1998 styles like Neo. It didn't start to become available on the more classic styles until mid to late 1998.