Technically, no, you don't have to change the oil. Officially, in the
US, yes you do.

The USDOT, requires that vehicles using Freeze 12, Duracool, HC 12a and
other similar products must first have been converter to R134a fittings
and oil and be properly labeled.

However, as I have commented before, and Glen Smith echos, these other
freons are GREAT in Vanagons.