How to save old Messages on a Mac? i have all my old conversations from Messenger on my Mac (and not on my iphone). As I want to put the upcoming High Sierra on a previously deleted Mac instead of simply updating my Mac I wonder if it is possible to somehow save the old conversations. Any ideas?
iMessage conversations appear to be saved in ~/Library/Messages.

To see your Library folder, you'll need to use Go > Go to Folder... (⇧⌘G) and then type in ~/Library. If your new Mac doesn't have any messages on it, you should be able to just copy the folder over (but make sure Messages is closed while doing it).

Make sure to back up the destination Messages folder before copying if you are worried about losing data (just rename it Messages-Old or something). If everything works okay, you can trash it afterward.