Before you ever perform a hard reset, keep in mind that you will lose ALL of the information that is store on your PDA and if you don't name exactly as it was before, you will have a harder time recovering that lost information. The easiest way to change the name on the device is to simply open the Palm Desktop software. Then, click on your USERNAME in the top-right corner of the screen and finally on EDIT. This will bring up a list of all users on your computer. Click on your name and then on RENAME or EDIT and give it the name you wish it to have. If the name is already in use, you will have to choose another name.
Now, if you are trying to restore the PDA after the battery fails or if you had previously hard reset it, you would need to hard reset it yet again and then select a name from the list on the next sync. You will lose all the information on the device so if you have added anything that wasn't on the previous HotSync username, you won't have that informatin.
You can, however, open your Palm Desktop software and export the information from one username and then import it to the new name. This would save you the time of setting up the device and waiting for your restoration sync to complete.