When you transplant you don't 'change' the soil; you are simply putting the entire rootball with all the old soil still attached in a larger pot with fresh soil.
The roots need to be able to keep growing as the plant grows larger.
Personally I distrust Miracle Grow soils because they are peat-based and contain fertilizers that you willl have a hard time getting the feeding schedule right because of. Oooh that was awful grammar, lol, but i'm too baked to go back and change it.
The Miracle Grow organic is not that bad if you add some extra perlite to it, but a quality potting soil is a wonderful thing.
You still will be giving liquid fertilizer during bloom. Flowering cannabis feeds more aggressively than the ferts in that soil will be able to provide for, and at the wrong ratios of NPK. The amount of Nitrogen in pre-ferted soils isn't great for flowering; it can lead to leafy growth and not as much bud.