My '93 mx-6 ls loses a pound of r12 about every 2-3 years. Currently it has pressure but is not very cold. Here are the choices I have considered so far rather than keep on paying the ever larger bill to charge it :

convert from r12 to r134a:
My Mazda dealer will convert me to r134a for ~$200. They do not change the accumulator out which seems sort of suspicious. They just flush, vac and swap fittings. Another mechanic shop will do it for ~$250 and recommends changing the accumulator (included in price). No mention by either of them about the expansion valve (orifice tube).

Freeze 12:
Should I do it myself with Freeze 12 off eBay. Seems like I could get by for a few years with this since my system holds a charge to a point. Downside is if I ever need real service, shops may turn me away since I would not have pure r-12 after adding the Freeze 12.