1. you will clog up your filter the day you do this, make sure to only clean the materials in dirty tank water. now is not the time to super clean your filter.
2. yes, you can switch out the gravel safely,
3. don't stir up the tank unnecessarily, it will get cloudy, don't worry about it
4. either take out half the water to a fish safe bucket or container(s) and pour it back in later,
OR, remove the gravel carefully, wait a few minutes or more,
5. then do a gentle vacuuming to remove some of the filth that will be on the bare bottom of the tank. YOU DO NOT WANT TO REMOVE TOO MUCH WATER, only a small water change should be done this day.
6. carefully pour the new well rinsed gravel into one end of the tank and replace any water you removed.
7. I'd wait a day before redecorating, because the tank will be cloudy and it's better and easier to wait
8. you can do more vacuuming later in the week as long as you don't try to overclean or change too much water. It's not the dirt that's as harmful as overcleaning.
9. avoid cleaning the filter except as listed above.