I did this to my first Tahoe about 6 years ago. I just plasma cut out the stock bracket and drilled new holes in it. I noticed a big difference over the 4x10's and it looks very clean.
I just did the same thing to mine yesterday, but I used a jigsaw with a metal blade.
Unfortunately, the Crystal Audio (made for SpeakerWorx, about 10 years ago) 6x9's I used have HUGE magnets, and are about 4" deep. I had to cut the factory grills, and mount the 6x9's through them and into the factory bracket, and use the grill covers that came with the 6x9's.
Huge, absolutely HUGE difference in the output over the stock 4x10's! I have my 4 channel amp set up so that only one set of inputs is used, and only one set of the sensitivity adjustments is active. With the increased output of the Crystals, I need a pair of 1-into-2 splitters, so I can run the amp with both sets of inputs and sensitivity adjustments, that way i can dial down the rears!