In my case, the air con simply stopped being cold. I can't recall how much it would cost, but I paid $160 or so to get the whole system converted to R134a. The ED introduced R134a as standard, so doesn't need converting.
I get that hissing noise too - I hope it's normal.
Jack Travis, E-Series Acting President
1989 Ford Fairmont Ghia turbo/GL replica (underway)
1994 Eunos 800M
1990 Ford Laser S