Poly does not adhere well to other things. I can relate my annual "table from Hell" circa 2004. The owner wanted to make her table "a little more glossy" so applied poly over a brand new table. (Never mind that a few minutes of buffing by hand would have accomplished the same thing.) When I got there, nearly 1/4 of the table top had finish peeled off. The rest I stripped off with extreme difficulty.

There's a 99% chance that the finish is lacquer unless it's been poly-ed by some DIY type. So, I see two good options for you:
- Clean thoroughly, scuff sand (just enough to dull the finish), and brush on a coat or two of brushing lacquer (Watco and Deft are two commonly available brands).

- Strip the finish using a chemical stripper, then proceed with staining and applying a finish of choice (you could successfully use poly here). If you are not an experienced varnisher, a wipe on varnish, like Arm-R-Seal wiping polyurethane would be a commonly available choice.

Sanding down the the color (whatever that is) is also a bad idea.
* It's very difficult to sand down evenly.
* It's very difficult to remove all the finish by sanding, you might not see it until you try to stain it.
* You stand a real risk of sanding through a veneered top

Finally, "poly" is not the be-all and end-all of finishes. This is one of the things I relay in my finishing classes and posts on forums like this. It's cheap, hogs all the shelf space, and is abrasion resistant. That's about all the good qualities. The bad qualities are it ambers significantly, has poor adhesion, can be difficult to apply smoothly unless you know how to do it, has low UV resistance, and is nearly impossible to repair.

Thanks so much!