I used wrong wording in my first post, you can't 'restore them back' but you can view them from your media card.
If you have a saved convo from said contact, roll over thei name and hit the bb button, from there hit 'view history'
Posted from my CrackBerry at wapforums.crackberry.com
Before this will work, make sure you change BBM settings to again point to Media Card, as it will only look for history where it's currently saving history to. If you choose phone, it'll show you any history you've saved to phone. You need to change it to media card to see it from media card.
Thank you both, Fubaz and Chrispipe, you've both helped. And somehow, this time on a second attempt, my histories showed up. I had looked after the OS was initially updated and had not found them then...