It usually is pretty stressful for the fish, after all, you are literally changing their whole world! Plus, you are going to kick up quite a storm in your tank. Having said that, people re-aquascape all the time and if the fish are generally healthy they typically get over it fine.
A few things I would be mindful of though..
1. try and aquascape gently if possible. You are going to kick up a lot of debris off your rocks and sandbed and stuff from under the rocks. If you can lift your rocks slowly to try and minimize the amount of gunk scattered throughout your tank the fish will appreciate it. In fact, if you don't absolutely need to move the base rocks, I would suggest to leave them and just move around the upper rocks. After the rescape use some sort of mechanical filtration like a filter sock to trap the debris but be sure to check on it often as it will gunk up quickly!
2. watch out for falling rocks as this can easily crush fish. Once you start moving rocks around the fish will probably start trying to hide in and among the rocks. So keep track of your fish. You could easily end up lifting a piece of live rock and set it on the floor only to find one of your fish hiding inside of it afterward
Depending on the extent of your intended aquascaping it may be a good idea to remove your fish, but that in itself is pretty stressful too.
3. Don't leave rocks out of the tank for too long otherwise you will get some die off and then once you plunk those rocks back into the tank you will get a cycle, which is not good! If you really want to play it safe you should set up a temporary storage bin (rubbermade tote or trash bin) with a thermometer and powerhead. The next time you do a water change put some of that water into the bin and top it up with some new salt water. It is almost like a quarantine for the rocks. This way they have some place to sit while you're aquascaping.
Do you know what you want to aquascape too? Just randomly moving the rocks around? Trying to thin out the rocks? Need more rocks?