Hi,
The index files are built to contain a bright subset of stars selected at a given scale, AND "features" (micro-constellations) with a given range of scales. So you can't just drop stars, you would also need to select new features at a new set of scales, likely.
So I think your question boils down to, if you only have an index file, can you get back to a list of RA,Dec,mag, from which you could build a new index file.
One issue with trying to do this: index files don't always *have* the magnitudes of the stars, that's an optional thing.
The RA,Dec positions are encoded in a rather complicated way, so reading them straight out of the index files will be a pain.
You might try using the "query-starkd" program to "query" all the stars in the index -- eg, query for RA,Dec of 0,0 with a radius of 180 degrees, or something like that. I'm not certain that will work, but it might be your best bet.
cheers,
dustin