What's the water volume of your new and old tank ?
I'm guessing you just add tank height without changing the stand .
Be sure that your new tank silicone is fully cured,rinsed well and it doesn't have that silicone smell .Other than that it should be fine .
Keep current water % during your WCs in your new tank.
You can put them in 100 % fresh water but use drip acclimatization or pour a cup of water for an half an hour to avoid potential PH shocks.
Use the old tank once your fish are out to thoroughly mix and clean the sand from all the gunk in your old tank water so you can start with clean sand in your new tank .
Good luck and post some pictures once you are over with it .