Dex III to Dex VI

[Arnold]

I heard,read that there is no "real" Dex III left (maybe very little nos) what you get when you buy DexIII is a sort of "blend"..one of the "multi vehicle" fluids. Apparently patent rules from years ago. Even GM just uses DexVI even where Dex III was called for.
 I am thinking of changing my 01 to Dex VI. I was going to change the transfer case fluid..which was DexIII..and the GM dealer said that DexVI was the way to go..might do the trans too.
 Is siphoning out 1 quart of DexIII and adding 1 quart of DexVI..driving it a few thousand..repeating with 2 quarts..ditto..repeating with 4 quarts..then changing all of it plus the filter the way to go? I heard that is the best way anyway for changing trans fluid.

 Thanks..