Seedling mixes are usually pretty mild as normal levels of nutes can easily burn the new roots.
Once the plant has established a good root structure down low you should be fine as even if the hotter mix you're adding burns any new roots up high in the new mix, there will still be plenty down low to power the plant.
Generally the upper roots are considered feeder roots and the lowers more water roots, though I'm not sure there's all that much distinction. The finer top roots spread out and try to smother out competing plants for available light water, like early morning dew on the grass, so they are quite capable of sucking in water as well as the lowers at least on a soil plant.
Once the plant has established a good root structure down low you should be fine as even if the hotter mix you're adding burns any new roots up high in the new mix, there will still be plenty down low to power the plant.
Generally the upper roots are considered feeder roots and the lowers more water roots, though I'm not sure there's all that much distinction. The finer top roots spread out and try to smother out competing plants for available light water, like early morning dew on the grass, so they are quite capable of sucking in water as well as the lowers at least on a soil plant.