WoW Classic will be 64 bit guaranteed. If you've read the dev water cooler about their first 2 prototypes, you know that they actually use the current client with 1.12 data. (If you didnt read it, here it is: https://worldofwarcraft.com/en-us/news/21881587#1)

There is just no point supporting 2 different Client Versions if you can use the same with just different game files.

So we asked ourselves, would it still be possible to deliver an authentic classic experience if we took our modern code, with all its back-end improvements and changes, and used it to process the Patch 1.12 game data? While that might seem counterintuitive, this would inherently include classic systems like skill ranks, old quests and terrain, talents, and so on, while later features like Transmog and Achievements would effectively not exist because they were entirely absent from the data. After weeks of R&D, experimentation, and prototyping, we were confident we could deliver the classic WoW content and gameplay without sacrificing the literally millions of hours put in to back-end development over the past 13 years.