Since smartmlp did not indicate his/her location, maybe I should add this comment.

If you are not in a warm climate such as Texas, you MIGHT get adequate cooling with 134, but you as of today, you will spend more money preparing for the conversion than you will spend for the difference in cost of refrigerants. Additionally, by the end of this year, it is very possible that R12 and R134a will be about the same cost since R12 prices are coming down and R134a prices are going up.

In a not so hot climate a few years ago, you could build a case for conversion, but it makes less and less since as time goes by.

Have a great day,
Doc