Identifying paint type...

Hi.

Just moved house.

I want to paint a bedroom with a matt emulsion.

The wall is already painted and has a shine and a tacky/slimy feel if I run my hand over it.

Is this likely to be a vinyl silk or a silk emulsion? How can I tell?

I've got the feeling the matt emulsion won't stick, or bubble... Am I best to give it a light sand or use some sort of primer? I've used Zinsser 1.2.3 to good effect on a kitchen wall previously, but that will be pricey for a whole room.

Thanks