A 6" x 6" porch post would be a finished dimension of 5 1/4" but from here I cannot tell how yours are installed. You will ineveitably need to support the porch roof somehow when you remove the existing posts unless you can tell 100% that they are purely decorative and not structural.

I replaced these posts about 20 years ago, the old post's were rotting on the bottom due to being in a little weather for a long time.
I set up a small section of wooden cribbing, then with a smal 12 ton hydraulic jack supported each section as I replaced them.
I supported from the rail not the floor, gave me a shorter cribbing.
Remember I said each section at a time, I would not support the entire porch at once.

not me anyway