Can I put new 134a into an OLD AC system

The only way to do this is a full system flush and partial component upgrade for R134a. There will be enough oil residual mineral in the system and the two are incompatible. You will need an R134a expansion valve as the orifice size is different for the operating pressure or you will blow high side hoses. Reverse flush the system completely, replace the expansion valve and the dryer, add a high pressure switch, use ester oil and you should be good.

Thanks Joules...I'm trying to learn.

My understanding was that although the oils do not mix, they will not react...I assume I have very little original oil.  I'll probably try to flush just to be safe.

Expansion valve.  My understanding is that the expansion valve is like a tapered pin that moves in an out of an orifice to permit either more or less refrigerant to flow. That tapered needle is controlled by a capillary tube, the colder it gets, the smaller the opening. Since there is only about a 10% difference between R-12 and R-134a, the expansion valve being self-compensating really doesn't make that much difference. 

Is the information not applicable to the old Pantera system?  I had a 1991 BMW switched to 134a by just changing the low pressure charging valve, flushing and refilling with 134 (maybe with 10% less charge to compensate for the different pressures)...without any detrimental long term affects.

If I were going to do the conversion I would :

(1) Purchase a new compressor designed for R134a, verify it is filled with the right amount of oil before I install it.

Many folks simply flush the existing compressor and refill with the proper synthetic refrigerant oil. But all the rubber parts, like the shaft seals, are not compatible with R134a. Plus lets face it, that old York compressor is an old soldier. It has to be retired sometime. Its bound to be a leaker.

(2) If the new compressor requires different fittings I'd replace the hoses with new barrier hoses having the correct fittings for the R134a compatible compressor. However, if the new compressor IS compatible with the fittings on the original hoses then I would simply disconnect all hoses at both ends and flush them with solvent, blow them dry with air. I'd repeat this as often as necessary until I was confident the hoses were clean and dry.
(3) Flush the evaporator and condenser the same way.
(4) Replace the receiver/drier tank. This is something I was taught to do every time I break-open a refrigerant system.
(5) Replace the O-rings at all the fittings with R134a compatible O-rings.
(6) Folks in the refrigeration business say installing a high pressure switch is a good idea, so I'd do that next.

(7) Most R134a conversion kits don't have replacement expansion valves. People seem to get away with their conversions without replacing it. I'm not qualified or experienced enough to say it must be replaced, or not. I'm not saying replacing it isn't a good idea either. I'm just saying that a lot of people perform the conversion using the OEM R12 valve. However, if the replacement expansion valve is readily available, if it is a direct "bolt-on" swap that doesn't require changing fittings, and if the price is reasonable … I'd most likely say to myself "heck why not".

(8)  Evacuate the system with a vacuum pump for at least 24 hours.
(9) Fill the system with ≅ 80% of what was required for R12. If memory serves me correctly I think 80% is the right number.

Thanks George and Joules

Have you considered staying with R12?  On a recent Pantera project I had an AC system that seemed in a state that is similar to your description.  After careful thought I pumped the system down and added two cans of R12, and the system worked like new.  You can still find R12 cans on craigslist or eBay for about $25 or $30 per can.  The system still had sufficient oil to keep the York compressor in a healthy state.  R12 likely works more efficiently than R134A in a stock Pantera AC system.

Where does one find instructions on servicing the system ?  The service manual tells you to "service". Nothing more.

It turns out the system blows pretty cool...probably a little charge will work.

I am looking for R12...but I am seeing a LOT of 12a.

I agree with Steve, if the old system will still work with  R-12, just recharge/recycle with R-12.  Look for a shop that works on classic cars and has a supply of R-12 and a recycling machine.  They will not use the same machine on R-12 and R-134 to prevent cross contamination.  The shop will have to evacuate and recycle the R-12 that is left in the system (cannot legally discharge into the air), and add R-12 as necessary to get up to the specified charge.   

Converting to R134a seems like a lot of work (per George's description), and as of the year 2020, or 2021 model year, light duty vehicle manufacturers will no longer be able to use R-134a in their new automobiles. The mandate does not say what they have to use instead but there are very little choices to choose from. Most manufacturers will end up going with the HFO refrigerant known as 1234YF.