New and old phones can be activated / changed / swapped / switched  in place of your current phone at any time online. The process will take you through a series of steps to change out a phone and, if needed, it will tell you if you need to change your plan to match your device.  Please make sure you read and follow all steps during your phone switch.  Here is the Support article on changing your phone for your reference.

Simply log into www.sprint.com with a profile set at the account level. If you are not sure if your online profile is at the account level, here is a blog where you can check, and if you are not, take step to change it. Next, scroll down to your devices and select the drop down to the right of your device which say Manage this device.  Select the page saying Activate a new phone.  Follow the instructions, making sure to read steps for each part to change out your new or used phone.

Often customers ask if switching to an older device (perhaps a current device is broken), will that affect their upgrade date. No, it won't. Your upgrade date is based on the last time a phone was purchased and a contract signed.

You can also go to this activation link. This phone swap page has instruction on starting the process, a way to log in and an activation instructional video for your reference. In addition, at the bottom of most pages, you can click on the Activate a new cell phone link under Manage, and it will take you to the starting activation page.  This link will work for both new and older devices.