I was recently going to upgrade a Cisco image on a switch but wanted to keep the webinterface etc and to update to the next image release which was a tar file. The easiest way of doing this and upgrading the image … simply to enter one command..
Switch#archive download-sw /overwrite tftp://tftpip/file.tar
Basically you can specify either your TFTP or FTP server where the TAR file is and download it to the switch. Once this is downloaded it will overwrite anything on the switch. Most Cisco switches dont have enough space for an additional image so the overwrite parameter is useful. After the upgrade is complete , simply issue reload. If you prefer to reload it at the same time you could include /reload parameter after the overwrite.
Switch#archive download-sw /overwrite /reload tftp://tftpip/file.tar