I think that if you want to see the current messages, you need to use these files :
~/Library/Messages/chat.db
(same path on Mountain Lion (ML) and Mavericks).
I have not tested myself, but likely if you copy these files (possibly 3 files : chat.db, chat.db-xxx, chat.db-yyy) from ML to Yosemite your messages will show up in Messages.
Backup the current files (copy them to the desktop for example) on Yosemite before replacing them with the ones from ML.