Not using an undercoat with solvent based paints means it may not stick. It may seem okay, but the next time you come to paint it comes away in sheets when you get sand paper anywhere near it, this also counts for the so called one coat gloss such as Crown solo and Dulux Once.
IF you steadfastly flat the old gloss down using a wet sanding method, or use a de-glosser then you may get away with it, but the time taken is probably greater than undercoating anyway.
Eggshell paints are different (solvent) you can with some of them paint straight over old gloss after light sanding and washing down with sugarsoap. But even this is dodgy practice these days as the manufacturers are forced to meet new criteria.