Can I reload old iMessage conversations on a new device (iOS and OS X)?
I just transferred all my chat history by copying ~/Library/Messages from an existing (OSX 10.8) machine to a new one (OSX 10.10).

Couple of things to note:


Quit Messages on both machines before you do this
After copying over, delete chat.db-shm and chat.db-wal from ~/Library/Messages on the new machine
Empty the Trash. I found that there was a process called 'IMDPersistenceAgent' that was holding on to the files I deleted. I killed that process ( sudo kill -9 pid in a terminal). To find out which process (and the pid) is holding on to those files I used the excellent What's Keeping Me. It found the culprit process, but I wasn't able to kill it from within it (and hence had to resort to killing from a terminal).
Start Messages. Now it might seem that you did not get the history back, but you did!  Try scrolling up for any existing conversations and you will find that the history has been restored.  If you were to start a new conversation with a contact that is not yet visible, you will find the old conversation history is now visible. 


I am happy with the above and am not too worried about the list of open conversations not being restored as long as the history itself is there.†w