Your indy is wrong, I would go somewhere else for a/c work from now on, or insist that he use R12.

To begin with, you have to flush the system to remove all the old mineral oil, then replace it with the newer synthetic oil that is compatible with R134a. Then, you have to change all of the refrigerant hoses in the car, or else you're going to need to recharge the system every time you turn around, as the unlined hoses in an R12 system will allow R134a to escape easily. R134 requires lined/coated hoses.

And if the cost and aggravation of all that isn't bad enough by itself (it is), then be prepared to have an a/c system that only works half as good as it used to. R134a is less efficient than R12. The 134 will never come anywhere close to the performance you are used to out of R12. If you live in a hot climate, you will not be happy with the result at all.

I would absolutely NOT make this switch. It's not going to give you the performance you're already used to. As another member said, it will only work maybe 70% as well as the R12 did, and I think he's being pretty generous when he said 70%. I've had both 12 and 134 in my W126, and I would personally say the R12 worked closer to 2x as good as the 134. And the part about being able to just suck R12 out and put R134 in, without doing anything else is just wrong. I mean, yes, physically you can do that, but it's never going to work right. Your guy doesn't sound like he knows what he's talking about when it comes to conversions.