1990 Subaru Legacy Freon to R134a

I've got a 19-year-old car with a 19-year-old A/C system, which means it was made for freon and not R134a. I want to make my A/C work for next summer, but it's too expensive (and probably illegal) to recharge with freon, so I want to know what I need to do to make the change. The compressor (and the clutch on the compressor) is just fine, the entire system has been purged and is completely leak-free.