No, you do not "just dump" the R-12. In this case the system was empty so it is a non issue. Also, there really is no reason to convert if there has not been a freon loss. To vac the system the shop would use a recovery unit so the freon would not escape. The R-134 is different in charging the system, it requires weighing the quantity.
There is nothing wrong with R-134 - it cools very well and is used in new cars.