Dirt? Naw man, I can't imagine a Fluval Edge 12 gallon with dirt. Ooh the work needed for that, shudders!
Plus, you got to wait for the dirt to age... blah blah ... not worth it. Sure there be some guy coming in this thread and telling us that DIRT is the greatest thing ever! Maybe in the 70s or something, we got options now. Just buy ADA Aquasoil or any of those substrates, they're all baked dirt-based balls. There is even talk of people using plain old Bonsai soil, but I'm lazy and I just went with the best looking substrate ever to bless the hobby - Seachem Flourite Black. Some people like the look of brown balls in their tanks, I don't.
Anyways, you can totally change the gravel. You got Prime, Plants and a mature Aquaclear sponge, I'm sure you will bounce back in no time. Just do water changes if you find your readings are going too high. Your plants will have a feast!
EDIT: I hope there someone from Seachem or ADA who reads this and say, yo send this kid a bag of our dirt.