Flush every component. The mineral oil is not compatible with r134.
If you are using a Sanden, you might consider doing a POA eliminator kit so it becomes a ccot system. Poa systems are frosty, but I don't think a poa would be a good fit unless using an oversized compressor like an A6.
You are correct, POA valves can be adjusted for r134 pressure, and a 134 spec expansion valve could be used.
New hoses should be used, especially for 134. The old hose has a molecular porosity not good for 134. Orings need to be the green ones. Ester oil can be used for either r12 or r134. Pag oil is only for 134. Mineral oil is only for r12.
The part about original systems that scares me most is the controls and vacuum diaphragms.
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"...ridge reamer and ring compressor? Do they have tools like that?"
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