So this one is not exactly pulitzer prize material, but it took me a bit to find the correct answer as I was not searching with the correct search terms. After all I am a systems administrator by trade, not a network administrator, so you are going to have to give me a break if I don't speak your special little Cisco Router/Switch language.
Anyway, I actually ran into a device that had a large enough flash that it did not ask me if I wanted to delete the existing IOS image during an IOS upgrade. Wow, two IOS images in flash, how novel.
So here is the process for changing your boot img.
Obviously you need to log into the switch first. Needless to say you must turn the device on first as well.
Check out your flash images via the command below.
Directory of flash:/
2 -rwx 1751867 Mar 01 1993 00:06:57 c3500xl-c3h2s-mz.120-5.wc3b.bin
3 -rwx 556 Mar 01 1993 00:00:41 vlan.dat
4 -rwx 25 Mar 01 1993 00:00:35 snmpengineid
5 -rwx 2068 Mar 01 1993 00:05:39 config.text.old
7 -rwx 1811552 Mar 01 1993 01:17:44 c3500xl-c3h2s-mz.120-5.WC17.bin
8 -rwx 43 Mar 01 1993 01:24:40 env_vars
9 -rwx 2077 Mar 01 1993 01:24:56 config.text
3612672 bytes total (40448 bytes free)
Then in config terminal mode run the following command – subsitute your bin file name.
s-3500-2(config)#boot system flash:c3500xl-c3h2s-mz.120-5.WC17.bin
My tftp server is a Linux box, setup instructions are HERE