Hi,
If these are iMessages in Messages 7.x.x in Mountain Lion they are held in an item called Chat.db in ~/Library/Messages
If you have Time Machine and go to this Folder (Messages) you could replace the chat.db item with an older one that might have "history" that goes back further - but you will lose the more recent items.
There can be issues if the items was replace earlier as this will lose the "history" in the previous chat.db item.
The Messages beta did the best of using older Saved Chats compared to the Messages 7.x.x versions or the Mavericks' Messages version 8
To be clear the Save Chats and the "history" that the chat.db database items in fill are different.
For iMessages it seems the app relies on the chat.db items.
Things were moved fro the Mavericks upgrade which also impacted on this.
8:32 pm Monday; April 7, 2014
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad