Gravel changing?

[Fishnewb101]

We currently have a 15 gallon tank if your wondering it has the tetra whisper internal 20-30(40 ?) and a undergravel filter with charcoal on the tubes using a pump now before everyone starts hating on the UG it's working great for us we currently have 5 red eyed tetras, 5 black stiped with black tailed small angel fish looking guys? I don't know the name for sure, 2 white cloud minnows, 2 African dwarf frogs, 2 ghost shrimp, and 3 mystery snails, yes it's overstocked but the water is in perfect condition and everyone is happy and healthy with our filter setup and also have a few live plants and do weekly water changes to keep everything in check now mom wants her gravel to be a little larger and look more natural ( we currently have the natural colored rock in the 20 pound bag from wal mart) and she wants the larger same colored rock I believe it's the coral nuggets in a 5 pound bag from wal mart and I'm wondering how were supposed to do this as I'd have to take all the gravel out take the UG filter out and the rocks that got through out put the UG back in and put the newly (rinsed gravel in and I imagine that in doing that the fish would be very stressed out, not to mention all the good bacteria that grows in the gravel would now be gone unless that's ok because the filter would still have bacteria and the UG would still have bacteria under it as well so I'm wondering what you guys think would be the best least disturbing method of doing this no we don't have a backup tank, although I am Recieving my new hawkeye 5 gallon soon and I could possibly stick them in there until I finish this or maybe get a 5 gallon bucket and they'd be fine in there? thanks guys for the patience