You will see more of a difference if you switch between balls that have the same cores but different covers. With coverstock/surface dictating the majority of reaction on the lane that option will be better for bigger changes.

Switching between balls with the same coverstocks and different cores isn't a terrible idea either. One example off the top of my head is going from the Code Black to the Hy-Road Pearl. A pretty big difference, although Storm might be the only manufacturer that you see a ton of that with. It's rarer for the others to use the same cover with significantly different cores. Or at least them actually telling us it's the same cover with different cores.

Transition is tricky though and not always just balling down/up. I wouldn't make this a big consideration for building a bag. Sticking to having different shapes/lengths regardless of a certain set of cores or coverstocks is a better bet in my opinion.

It depends on the transition you see. There is no set best way, what might work one day might not work as well the next.

It should definitely be a consideration to have multiple options with different shapes

Also if it's carry down vs ball losing too much energy up front and not finishing as strong down lane.

I've had this a lot recently.  Ball doesn't hook off my hand, or ho through the face like you'd expect when the fronts dry out, but it's not making as strong a move down lane and is leaving 10s that it wasn't before.

I move my feet a few boards in and like magic the ball is back to recovering down lane like before.