Steve
It is highly likely that when you start LR3 and tell it to open an LR2 catalogue, it will convert it in its entirety and create a new catalogue in the LR3 format. Your LR2 catalogue will be left untouched, and you'll still be able to open that old catalogue in LR2 (if you ever want to do so). That's what happened with LR1 to LR2, and it's very probable that it'll be the same this time. In other words, you're right to be concerned but I wouldn't worry.
John