It is possible to replace only the digitizer, however there is a lot more work that way, as the lcd and digitizer are secured to each other with adhesive. Which you will need a lot of time with a heat gun, solvent, and scrapers to remove. Afterwards it is a delicate process to replace, as it will need to be replaced and reglued, UV cured (depending on adhesive) and reassembled.
I'm getting ready to do this with my phone, but hindsight being 20/20 it's worth the extra 70 bucks to get the one already complete with lcd...