While a vessel is on suborbital trajectory, you cannot switch control to other vessels, even after going back to tracking center. This creates an issue as I jettisoned my only on-board Kerbal in an escape pod. So, I can't control the escape pod and save my Kerbal, I'm forced to watch him die and pilot an empty vessel to the ground. I'm not entirely sure whether this is an intentional decision or a bug, but either way it's something I used to do semi-frequently in KSP 1.