Messages and preinstall posts

What do you mean by messages? email? imessage? I have never "lost all history" upgrading a Mac. Ever. So I'm very confused about what you are asking. Is "back post history" some sort of reference to email?

Let's assume you decide to install Mavericks on your "real" computer once it comes out. I don't see any reason you should worry about losing anything but you do know how important it is to back everything up, right?

I have a cautionary tale to share...

I decided to upgrade my wife's mini to Lion. I inserted my homemade lion install USB stick and started the upgrade process. At some point, for some reason her Macintosh HD went away. It was just gone. I could have fired up disk utility and gone poking around for it but I decided to go through with the install anyway to a fresh new partition over top of where her stuff should hav been, including digging up the original dvd that came with her Mac, installing a blank OS then upgrading to Lion. I planned to migrate from her Time Machine backup on our Time Capsule. It turns out the backup had become corrupted and was useless. Lucky for me I had crashplan running on her machine and everything came back.

Upgrading OSX is one of the safer more mundane things a person can do to a computer but I still recommend having a fresh new backup just in case something goes wrong like a power failure in the middle of the upgrade.

 

Without searching I don't know where the past history is stored for Messages.app.
I would copy any old Application Support folders and any .plist files you can find. Hopefully you will get your history back once they are copied.
If you upgrade you won't lose any history.

 

Like most apps data is kept in ~/Library/Application Support
I don't use messages, so I'm not sure if old messages are kept when updating. I don't see why they would not be updated, especially if there is info in Application Support for the app.