There are specific instructions in the Drive Upgrade Guide for doing this. They are down near the end of the guide, section titled "Step 7: Copying files (special circumstances only)". They include detailed command line input instrunctions. In short, you simply copy the entire contents of the FIDS folder from each of the 30gb drives to the single FIDS folder on the single 250gb drive.

In theory, the fid numbers should not overlap.