These paper caps are typically safety rated (for failure mode, this is what the "X" or "Y" or "X2" means), and the poly ones frequently aren't rated the same way. Be cautious.
http://www.kemet.com/Lists/Filestore/EvoxRifaRFIandSMD.pdf
http://www.kemet.com/Lists/Filestore/EvoxRifaRFIandSMD.pdf