I don't think the big issue isn't whether the fluids mix, hydraulically speaking it doesn't matter.

The problem might arise with fluid compatibility with the seals and soft parts. Probably a bigger issue when new fluids came out in the 70s, not very likely with today's rubber compounds.

If I was changing over, I'd make sure to clean out all the old fluid, to the point of disassembly or replacing parts. One doesn't have to, but why hamstring the system with old fluid?