Ok, step by step:
First, if you don't have one, get a gravel vac.
Second use it, I have live bearers and they are seldom ever caught in the syphon. They get caught in the filter much more often.
Third, remove 1/3-1/2 of the water volume using the gravel vac. It is supposed to be self priming meaning that you raise it up and down really fast until the water starts to flow and lower the tube, which should be in a 5 gallon bucket. If it doesn't work then suck on the end of the tube until water starts but try not to get too much of a mouthful.
Next, refill you bucket with the same temperature tap water adding dechlorinator to it as you fill.
Next, if you want to change some gravel, now is the time to do it. If you follow the post from the live bearer forum and cultured the gravel then replace half of it., If not, then only replace 1/3 of it.
Then refill the tank with the water from the 5 gallon bucket.
You don't need to change out the gravel for molly, sword, guppy or any other live bearer I have ever heard of. They are very agile and fast when born and they will run from the gravel vac. If they don't and get sucked up, simply catch them and put them back in the tank. You shouldn't worry about it anyway with the absolute huge numbers you will have. I have 1-3 that survive from ever batch even though I never remove any of them from the tank. If you want to raise all of them, then get a net breeder and put the prego female in there with some plants in the bottom so the fry can hide. If you plan to raiase large numbers of fry, then a separate 10 gallon is a better choice for the large numbers you will be dealing with.