You can stain over stain using gel stain. You can go same color or darker but not lighter.
If you want to go lighter you will need to and down to raw wood.
Purchase the Rustoleum kit for about $100 as it has everyting you require in the kit and has enough for most kitchen cupboards.
When we did ours in the eighties, we took them down and stripped and sanded them in the basement (winter time, couldn't do it outside or in garage). We like the way they looked after taking the old stain off, so we just cleaned then up and sealed them, but after stripping them, you will want to sand them and clean them before you restain and seal them. Mark your doors and cupboards as to where they went so that you can get the right doors on each cabinet. Clean up or replace hardware if necessary.
You can use gel stint save you time on sanding. :) You can check out my guide on how to apply gel stain here. Hope it can help you or give you some ideas!
Take one of the cabinet doors to the paint department of the home improvement store and ask them to select paint/stains and other finishes and application tools that will work for your kitchen. They have the best selection, the most up-to-date materials and most important, the most informative sales staff to get you what you need for a professional job.