From what we've learned in the last 5 years since the Salearea Guides were first posted, this seems to be the best current timeline of the serial number changes:
The Made In USA and Made In The United States creeds combined with the all-numeric xxx-xxxx serial numbers started in early 1988, although a few stray NYC creeds have been found in 1988 bags too.
The 3+4 numeric serial numbers were MOSTLY replaced in 1990 by the 4+3 numeric serials but a few 3+4 serials are still found in bags that can be dated to 1991 and 1992. The 4+3 all-number codes continued until the end of 1993 / beginning of 1994.
I doubt if every single stamp would have been replaced exactly on the first of the year, nor were the serials and creed stamps always changed at the same time. We've seen the old "scars, scratches..." creed stamp continue to be used as late as 1996 while almost all the serial numbers were updated to the alpha-numeric replacements that included the style numbers and production codes early in 1994. So Coach didn't or couldn't force ALL these changes into effect by one specific date such as January First of 1994.
Even if it was practical to change the information in the Guides, replacing it with what I just posted above - and PLEASE let me know if anyone believes the dates or information is muddled! - would just lead to even more confusion. What's already in the Guides can be hard enough for many people to process and remember without adding anything even more complicated. Like with everything else Coach does, there are always going to be exceptions.