1. Select WoW.exe
2. Right click on WoW.exe and click on Pick Link Source
3. Right click inside the window folder and hover over "Drop As" You'll get either Hardlink or Symbolic Link.
4. Select Hardlink
5. Rename WoW-64.exe into WoW-64-Backup.exe
6. Rename the Hardlinked WoW.exe into WoW-64.exe (You'll see the weird symbol behind it)
Tada! You are now forcing the new battle.net launcher to always launch the 32bit client because you are saying to the Windows that WoW-64.exe is WoW.exe instead.