I think the change started shortly after the meatball logo was replaced with the current logo and livery (I believe this was around 1992-1993).

Around this time, CO only had a few 747-100's and 200's, that they acquired from PEOPLExpress during their merger. They also had around 8 or so DC-10-30's. These were the only two aircraft types that were configured for F/J/Y class. They also had some DC-10-10's, which were configured for domestic F/Y and included a pub area which was between First and Economy.

I think the only routes that were actually flown "scheduled" with a 3 class airplane were EWR-LGW, IAH-LGW, EWR-ORY, SEA-NRT, HNL-NRT, HNL-AKL-MEL, HNL-SYD-MEL, and then sometimes HNL-GUM, HNL-LAX, HNL-SFO, NRT-GUM, NRT-SPN, IAH-LAX, and random domestics. Also, depending upon aircraft utilization, you could end up with a F/Y/pub version DC-10-10 on any of the above routes.

Around the same time, CO decided to give up on Australia and New Zealand, and discontinued service to those countries.
The only thing that I remember about First Class on the 747, was that the seats were kind of a beige-ish suede material and were definetly far from being the fully reclining flat bed suites that you see in First Class today. All meal service was plated in the aisle with multiple tiered carts, and presented on china without using trays.

At the time when BusinessFirst was rolled out, I would say the majority of the customers were happy, because the new BusinessFirst seats were a huge upgrade from the old 3 class First and Business Class seats (they were more like the First Class Seats than the Business Class seats.) For example, before the elimination of First Class the 747 had 16 Business Class seats upstairs in the hump, the new BusinessFirst cabin had 8 seats upstairs. So, it was a huge improvement in space and seating, plus the seats featured IFE, which was main-screen only before the re-configuration.

The only other thing I can remember was that EWR-IAH upgrades were not permitted for anyone, including Gold Elites (which was the top tier back then).

Hope you find this helpful. It's hard to remember things that happened almost 20 years ago.