You need to be careful with pre-fertilized potting soil as the NPK ratio may not be the best for growing tomatoes. You may also have read about many people getting lush green growth without any flowers or fruits.
My current favorite bagged potting mixes are Pro-Mix ultimate Organic and Gardeners Gold Organic Potting Soil (make sure it says potting soil --they sell compost/soil amendment in bags with very similar packaging). I don't know if you can find those where you are, but I thought I'd mention it.
Personally, I wouldn't "treat" the soil, but that's because I believe in living soilfoodweb in the soil. When you have a complete community living in the soil, fungus gnats, as well as other pest bugs are minimized, and I put at least one earthworm in every container --more for larger ones -- my living soil gardeners to condition and keep the container soil from getting compacted and keep the plants fertilized.
I suppose if it's fungus gnats alone that you are concerned with, in sterilized potting mix, then adding gnat controlling Bt to the initial moistening water (you do moisten your potting mix before using, don't you?
) might be the way to go.
To repot, lay the planted pot on it's side on a sheet of cardboard or --even bette for multiple uppotting-- flexible plexiglass or other waterproof stiff but bendable flat panel. Slide the plant and potting mix onto the panel. Put sufficient moistened soil mix in the bottom and one side of the larger container, then, rolling the sides of the panel to fit, gently slide the plant and soil into it, and while supporting it with the panel, add more potting mix on the other side. The plant should be sitting slightly higher than you want at this point. Now pull out the panel. Fill in and add more mix to the sides as necessary, then pick up the container slightly and drop it on the ground to thump and settle the mix. You may need to top with more mix.
When up potting tomatoes, I always barely line the bottom of the new pot and drop the plant and soil ball in so that at least some of the lower stem is buried. Clip off the lower leaves that will be buried or will touch the soil surface. I drop them in the bottom of the container to be buried and break down but you can throw them out or put them in the compost if you prefer (note that tomato leaves can be used to make pest repellant as well). Tomatoes will grow roots from the buried stem, establishing a larger root system. Be aware that this doesn't work with all plants and some plants are very sensitive to being buried above the original soil line.