1994 was apparently a mixed year. My '94 Ranger 2.3L has factory R134a, identified as such by the evaporator housing sticker and tags on the lines. BUT, it also had a small "Caution R-12 destroys the ozone blah blah" decal on the driver's glass. Earlier this week I looked at our 1994 Mazda B4000 company truck at work after it came back from getting its AC system fixed. It is plainly marked as an R-12 unit on the evaporator housing, and it still has the "Caution R-12" decal on the driver glass. So, two 1994s, one R-12 and one is R134a.
It's been two years since I converted my 1990 B2, so I'm working from memory here. When an R-12 system is converted over to R134a, EVERY o-ring in the system must be changed; old black ones to the green ones. I would have to look it up, but I believe that the reason is that the original o-rings are not compatible with the oils that have to be used with 134a. You also have to flush out all of the old mineral-based refrigerent oil. There's two refrigerant oil options for use with R134a; I remember that one is PAG oil, which I used as traces of mineral oil wouldn't adversely affect it. The other oil could not be exposed to mineral oil, or some kind of awful, terrible thing would have happened (reaction creating sludge, maybe, I think).
Edit- Ester is the correct oil which I used, not PAG.
On my B2 conversion I started with a new compressor, accumulator/drier and orfice tube. All of the other components came out of truck, were cleaned and flushed with the special gun and chemical (about 30 bucks, I think), and then everything was installed with all new (green) o-rings. In the Ford shop manual, I believe it was, I found instructions, that when replacing R-12 with R134a, to use 10% less than the R-12 charge weight. Always charge by weight, the exact weight specified, and add the exact amount of oil, as specified.
The conversion is still working perfectly, blowing cold air in the mid- upper 30s degree range. When I converted the AC I also did the Explorer radiator upgrade. It was well worth the expense and effort, but you can't do a conversion halfway and expect it to work right or last.