If your tank is fully cycled, and your filter was ready for the discus, then i am sure you could get away with that. But i added 4 to my 360L, and i had to do daily 50% water changes for the first week until my filter was able to deal with the amount of waste being produced - Discus poops are huge!
Now i do a 50% water change every Sunday. I have yet to have any problems.
Water changes is a personal thing. People will tell you different things all the time. What works for you is best. But what i was told is that having a constant stream of new water is better than large changes, say once every 2 weeks.
So a mix of practicality and logic from above, i adopted 50% each week. I make sure the water is the same temp, but i know the PH is different. This has not bothered the discus thought.
Hope this helps