I've been there before multiple times. If you plan it out and do it right would can make that upgrade with no issues.
Here is how I did it...
-Make sure you have plenty of saltwater pre made (more then the extra water volume you need, have enough for a few water changes in case you need to do a water change the next day)
-If the tank is going in the same spot you will need to remove all the corals and fish, a spare tank or anything that will fit them will work.
-Each time I have done the tank upgrade I have went with all NEW sand. I always tossed the old stuff.
-Sit down and write yourself a list of everything you need to do in the order it will be done.
-Review the list mentioned above and make sure everything is right. Now when you go to make the swap you don't have to think about what to do. You just do step 1, cross it off and repeat till your finished. This way you don't get half way through and be like damn I should have done this first. Makes the work less stressful.
The list and preparation are KEY! It might sound silly but put EVERYTHING on the list from start to finish.
I moved from a 10 to a 20 and also doubled our water volume multiple times with larger sumps doing it this way. I also moved our system (5 different bodies of water) without loosing a SINGLE fish or coral. In the next month I will be swapping my 20 high for a 93 gallon cube! I'll be making a list before I start for sure.
Last thing, TIME TIME TIME. Leave yourself plenty of time. Don't start late in the day in case for some reason it takes longer then you thought. It is surprising how long tank moves and upgrades can take. I think under estimating the time it will take and lack of proper preparation are the two reasons why most people have issues, lose livestock and crash systems during moves.