R12 to R134a conversion compatibility?

According to my Haynes manual, there is an explicit warning to not add R134a to unmodified R12 systems, because the accumulator and pressure switch must be changed, which was also the recommendation of a mechanic. However, all literature and product information I have read omits this requirement, and they've even said that all you really need to do is install the Low line adaptor, and change refrigerants. What's the danger in not replacing the accumulator and switch? If I had the AC lines vacuumed and installed R134a without changing the accumulator, then what would be the drawbacks? I'm particularly curious about system performance versus time. Anyone have any experience on this? Vehicle is a 1989 Ford Taurus. I can obtain the accumulator from a junkyard and get new seals, but is it worth it if I plan to change the engine about 2 years?