Sure, you can switch between Sprint phones at any time. Probably the easiest way to do this is use the online ESN changer in your Sprint account online, however this can also be done over the phone with a CSR. There is no fee to do this. When activating the new phone for the first time, it would probably be better to do this online, as I have heard some customer service reps have charged an activation fee on the first activation of a handset.
The only issue you might encounter is when switching from a Vision (1X) to a PowerVision (EvDO) phone, and vice versa. Swapping between technologies in this fashion might necessitate a plan change, or at least this was the case on the older SERO plans.
The Vision to Powervision is sorta hit or miss some people have switched between the two no issues (myself included) but you can't do that online and will have to call in. depending on who you get they may try and tell you that you have to change plans or they may just go about "add notes to the account" and send you on your way no problems and no changes