I'm strictly a layman in thsi area, but if they converted € and put even more złotys on the market wouldn't that cheapen the Polish currency even more?
No, because they're effectively taking złoty off the open market. The less złoty in circulation, the higher the price for one. There are issues with this though - which is why it's not used to massively manipulate the exchange rate. The Swiss lost a huge amount of money trying to stop the CHF from going below the 1.20CHF/1EUR barrier, for instance.
Where it gets interesting is when you look at the real value of the złoty - it probably should be somewhere around 3:1, but no government in Poland would support it there as it would destroy the Polish export industry.
One interesting aspect is that it seems that PiS intend to do exactly as I predicted before - they won't change very much from what PO were doing economically beyond extra taxation.