If I'm not mistaking roots of MJ do not really grow during flowering anymore. All transplanting would result into is a bigger nute/water resevoir, which can still be beneficial to a rootbound plant because you'd have to water less often and thus the soil's humidity would be more stable. Transplant big plants is troublesome because it is difficult to get the rootball out gently, wether in flowering or not ; a small plant 5 weeks in flowering may be doable.
the roots do grow during bud stage, maybe just not as fast. I only veg for 3-7 days @ 5 weeks plant is usually btween 1.5-3 feet tall. not too big, and easy to transplant.