I second what's said ^up there^. It's retro-active, switching it off only disables future comments, anything before that will still be there.

I'm just about to post in my journal to say IP logging is off so it's safe, I'm really pissed about this, yes, the info has always been available, but seeing loads of IP addresses is just not the same as loads of comparable location details and some people are reporting some very detailed locations, not just the nearest metropolis, but small towns and suburbs - and even listing past locations so tracking movements - so NOT cool.