Ctrl S (Cmd S if on Mac) saves the IPTC metadata and adjustments back to the images. You won't get stacks, virtual copies, assignment to dumb collections, edit history. You can import the files into LR2, though LR3 adjustments won't come over. It may be good enough.
Collections - well, it depends if they are smart collections. If so, right click each one and select Export Smart Collection Settings. That creates a file for each one, which can then be imported into the other catalogue. If they are dumb collections, you're out of luck.
John