Did you re-import the pictures (or "import" the old catalog), or did you let the LR installation convert your old catalog when opening the LR6 for the first time? In case of the former you might lose the history, in case of the latter it should all be there.
Your old (LR5) catalog should still be there. So a work-around could be to delete your new LR6 catalog and again try to open the old (LR5) catalog and let LR6 do the conversion again.
As the other posters it worked flawlessly for me, I got my full history back.