Don't take the chance that the old floor will stay on the ground. You should get a heavy duty floor scraper and get the old floor up, then buy some floor adhesive remover to help get the glue off the floor or concrete. Just follow the directions and continue to scrape it away before the new floor is put on. It's a lot of work but worth the effort. If there are any patches in the old floor, the new floor may get damaged by placing it over an uneven surface. If the floor is wood and not concrete, you might want to try to patch the old floor and install the new stuff on top, but if it's concrete, get to scraping.