For a new tank large PWC's are fine if you stay on top of them and if your water is alkaline.
However, it's possible to kill off your fish by doing a large PWC if you have an old tank, haven't done a large PWC in a long time and if the tank has been neglected and has acidic water. You could cause a massive die off in your tank by changing the water, if the water going in is alkaline and the water in the tank is acidic. What can happen is the ammonium in the tank's acidic water will be converted into ammonia when the alkaline water is added to teh tank. Ammomium is unharmful to fish and ammonia is toxic. This HAS happened before and I've heard of it being called "old tank syndrome". You will be perfectly fine changing out water in your planted tank, but just know that large PWC's CAN be deadly under the right circumstances.