It's fine to do it all at once, as long as you are making sure the water replacing isn't too different from the water you took out (dechlorinator/temperature/etc). I personally try to do water changes of about 30% at a time, just because I don't want to risk any large swings in parameters. (It's complicated but I use RO water and because I have things in my aquariums that raise PH/GH/KH, I want to avoid possible large swings on that front). But I think most people on here do up to 75% at a time without problems. So as long as the water going in is not drastically different from the water going out, it should be fine.