Welcome!
Personally I love a natural looking aquarium, but there's also a place for fake stuff too! Some people really love it, and some species of fish don't seem to mind too much. But it's definitely great that you're thinking about what your fish would want, rather than just the tank as just a decoration.
I also really like tanks that use elements of things like fake decor, mixed with natural things like dense planting. You can let your imagination go wild and have the best of both worlds! You can absolutely switch to sand while the tank is cycling, and it's much easier to do it now before you add fish. And if you ever want fish like corydoras or some other bottom dwellers, they absolutely would prefer sand that they can sift through. Sand is also a million times easier to keep clean than gravel is, so that's a huge plus. I've had both, trust me.
If you want to keep things like some fake plants or decorations for colour, you absolutely can! You can make a gorgous bright tank with some of those brightly coloured fake plants you have, and I see that a lot of the decorations have hidey holes, which many species will love! They don't care what the outside looks like, as long as it provides a dark little hide they can enjoy. Just make sure anything like that doesn't have any sharp edges, can run your fingers all around and over it, and sand off any sharp edges. Be wary of plastic plants too, they can often tear delicate fins. Some plastic plants are softer than others, so see how yours are. Silk plants are safer, especially for species like betta fish that have long, trailing, delicate fins.
For more ideas, I highly recommend this channel's "Thomas reviews your tanks" series:
Thomas also tends to go for more natural looking tanks in general, but can also appreciate a nicely done fake decor or themed tank! Themed tanks are a lot of fun. Going to be making one myself soon. He also gives a lot of tips on arranging equipment, stocking levels, planting etc, so you can learn a lot along with getting some inspiration.
To actually switch it over, you'll have to remove all the decor I'm afraid. Then simply drop the water level, scoop all the gravel out, and replace with thoroughly rinsed sand. When choosing sand, be aware that the very bright white sands are unnatural in a freshwater environment, and can even blind fish when powerful aquarium lights are reflecting off a brilliant white sand. The really intense fake dark black substrates are also unnatural, and can leave fish feeling vulnerable. Natural colours replicate their natural environment and make them feel more secure, so the fish tend to show their best colours too.
So don't let "natural only" snobbishness in the hobby put you off. If you like it and it doesn't harm the fish, then go for what makes you happy