Try polishing the lenses with a polish intended for acrylic (Lucite). This will often remove the yellowed outer surface of the lens, leaving it clear.
If you do need to replace the yellow headlights, it's only a 45 minute job for both. Open hoods, located the approximately 6 snap ring retainers on the engine side of the head lamp. Remove them, long with the headlight connectors, then pull straight out. That's it.
Another thing to try is regular automotive paste wax. Apply with a damp cloth, and use another damp cloth to wipe it off BEFORE it dries. Do a small area at a time, rubbing gently until the plastic clears. This also works well for hard water areas leaving a mineral deposit on the lenses.
You can also buy new ones on eBay for around 30 dollars a piece handling included. That is what I'm doing.
Another thing that I learned that is real simple and very effective is to remove the lens from the car or mask off the light with tape and newspaper. Removal is better because you can lay it flat and less chances of runs. Buy a can of clearacrylic spray pain at the auto parts store. It is also called clear coat. Take some 600 grit sand paper and a sanding sponge and sand the yellow off and any other imperfections. Don't worry about over sanding, 600 grit doesn't take off much. Once you have a nice frosty look to the lens take some rubbing alcohol applied to a rag and wipe it clean then allow to dry. Follow instructions on can and spray an even coat of acrylic over the entire lens. It might look milky at first but be patient and that will clear. Allow to dry then lightly sand again with 600 grit paper, just to give the paint something to stick to. Wipe again with alcohol the spray another coat. Once dry reassemble everything. Looks good doesn't it. This works for tail lenses too. It makes them as shiny as new. This is what the shops do and it only costs about $8.00 for everything. I have done several of my cars without any bad results besides when impatience gets the best of me.