If you are worried about stressing the plant when you transplant during flower, you can be ultra paranoid and do what I do. I take my pot (usually a 1 gallon) and I literally just cut the bottom off the pot with either a sharp serrated knife or a Dremmel. I then plop the whole pot right into the dirt. The roots will grow into the new soil. If you want to be super technical, you can actually cut the bottom off and then cut a line down the outside too, splitting the pot. Keep it together though and put it down into the new dirt and then slowly and carefully lift the old pot out from around the plant and discard. Ive done this a # of times with no ill effects and the plants definitely enjoyed it.

For me, the absolute tell tale sign of rootbound is curled under tips of leaves. Unless it does that, I generally assume its a different problem.