Brendan, it all depends the size of your bootflash to hold two ios images, you cannot rename running ios's, but you can leave it in bootflash, if you keep both images old and new you need to define which one will load with " boot system flash bootflash:newiosimage.bin " in cofiguration mode.

To find out how much space left in bootflash do

" show bootflash: "

The easiest way if you want to backup old ios simply copy it to a tftp server from 4503, delete old iosimage then tftp new image from tftp to bootflash and reload.

Jorge