You definitely should change to sand since you have cories. No, a total change won't hurt the fish, again, just be sure to use aquarium-safe sand. I was in the same exact situation when I started my tank. I see you're in Florida, go to home depot and get quikrete play sand, a 50 lb bag will be more than enough and it's gonna be cheaper than whatever you'll ever find at any pet store.
You'll need buckets to get the fish, plants and decorations out, put an air stone and heater (if needed). Remove the gravel and set aside. Remove the water (or the other way around, lol).
Get another big bucket, like the home depot orange 5 gallon ones, put some sand, maybe about a quarter of the bucket, add water and swish it a few moments, let the water go, repeat two or 3 times, then put that sand in your tank, and keep going until you reach the desired depth. 2 inches will do unless you have other plans.
If you want mixed media, as
@Byron said; you'll need permanent dividers to prevent mixing, the aesthetics are up to you.
Once you've got enough sand, put the plants back, decorations, fill with enough clean, treated water to bring the fish back in, then continue filling.
When I did it it was the first time I had done it, and the whole process took me less than 2 hours, all rookie mistakes included. Had I read about it before jumping in it probably would've taken much less, then again I tend to learn better by doing anyway.
I've experienced this loss of beneficial bacteria in a tank when switching out substrate, I've heard it referred to as a "mini-cycle". It can mean ammonia/nitrite spikes for a few days - a week after the switch, but it's easy to manage and ride it out if you're prepared in advance for it! It's not like starting a cycle from scratch, since you'll have the beneficial bacteria (BB) that you need in the tank already, they just need time to regrow the colony size.
My (well planted) tank had nitrite spikes for about four days. Meant large daily water changes, and using Seachem Prime, which binds ammonia and nitrite for 24-48 hrs to keep the fish safe between those daily changes. But because I was expecting it, meant I was prepared for it, so expected to have to do large changes for a few days and kept on top of testing the water.
That's a bit of a hassle, but it's only temporary! And easy to ride it out when you know it could happen. The cories will definitely appreciate a fine sand they can root about in - mine sometimes bury their heads right in there
Thank you guys so much for your input and all the great information you have provided me I will keep improving my knowledge everyday I am kinda extremely obsessed and want learn every single aspect of this hobby I can
@outofwater @Byron @AdoraBelle Dearheart