yes you can if you have a sprint.com account wher you log you will see your current phone then to the right it will say I want to:
Below that you will see "activate a new phone you click that and then you can do the ESN swap.
You may also select the "Got questions? Click to chat" option on the website as well. We are happy to assist you with prompt activation of your phone!
I have one question on the subject... when activating a phone online, say a phone you've had previously activated on your account, is there any fee for swaping esn's? I ask because I would like the ability to switch between 2 phones I have from time to time without calling in and without being charged for it.