As far as older coils having a max 150psi rating......I'm not familiar with that. So let's say that old coil sees above 83 degrees.....like after a hot air furnace, then it's above it max design rating?
Not necessarily, because the refrigerant will migrate to the coldest point in the system, and that dictates the system static pressure. Assuming no solenoids are present. If there's no liquid left, the pressure can't keep rising.
We're off point though. If the tag on the unit say the coils is " rated for use with R12, R22, and R502" then you shouldn't charge it with r410a unless you get the manufacturer to say it is rating it for that and they are willing to put it in writing. I know you can use the physical coil with out and problem but you shouldn't do it. Even if it wasn't a pressure issue, every manufacturer says you need to contact them to use any refrigerant other than that for which it was designed. That includes the replacement blends.
I gotta ask, why change to 407 right now? Dry 22 units are available like crazy, atleast around the midwest. R-22 prices aren't "that" bad right now, relatively. I have yet to have a customer, albiet 100% commercial, blink an eye at the prices. They understand it went up, and the increase hasn't been all that substantial. Maybe its different in other markets?