Yes you can switch at anytime
On your palm just do star 228, option 1. It will ask you for the 10 digit mobile number you want to activate it on and input your number. Most of the time this should work flawless, but if you do run into trouble just give customer service a ring
Switching to the palm pixi won't work via *228 because it has that free 5gb hotspot feature on that phone, and *228 fails if any feature changes.
You'll either need to do the esn change in your myvzw account ("activate phone" under more actions) or call customer service to swap them out. There's no charge for either method.