Short story is, we will provide an all-new downloader and patch; Mac players will need to delete their old WoWTest folder entirely before applying the next PTR update. This is true for players both on 10.3.9 and 10.4.x OS versions.
Windows users should be able to just log on to PTR and get the incremental patch; this added complication only applies to Mac players on this first PTR release, and should not recur with subsequent patches to PTR.