I don't necessarily disagree with BenzDoc about changing the parts although I'm not one to change good parts. I do, however, strongly disagree when it comes to converting to 134.

R12 prices are coming down rapidly while 134 prices are increasing at about the same rate. It may very well be that R12 prices fall below 134 prices this year due to rapidly declining R12 demand.

It will cost you more to do a proper 134 conversion than the difference in the two refrigerants and your system will lose about 15% of its cooling capacity. Additionally it is proven that statistically, systems converted to 134 experience failure within two years.

Good luck,