Along with other new evolution methods such as:
- Pancham -> Pangoro
- Eevee -> Sylveon
- Sligoo -> Goodra
They were all revealed by Gamefreak prior to the release of the games.
I couldn't really find a site where it specifically said they confirmed it but I found someone saying something that pretty much confirms it.
Do we actually know you need to hold it upside down, or is Game Freak just laughing at us?
Source of the quote above.
Hope I helped.