I wouldn't change that one at all, pretty sure it's Mica, it may have some stability issues if you replace it with Poly.
If I read this chart correctly the first Red dot indicates it a JAN Mica Capacitor,
Red, Black, are the first two digits and Brown is the Multiplier. Teh left side green dot is the voltage, 500v.
Here are a couple charts explaining the dot codes.
http://www.tpub.com/neets/book2/3g.htm
http://www.tpub.com/neets/book2/3g.htm
I've run into problems before replacing the square molded caps with regular Polys, even if they are paper there is probably something special about them, temperature, tolerance or whatever.