Just to add to Leo comment, the router might not be configured with a boot string, if not, it will boot the first valid IOS it finds in the default boot media. Second, if you don't update the running config (such as revising the boot string), you won't need to save the config (which should, I believe, prompt you whether you want to save the config when you issue the reload command).
Lastly, when you run a new IOS, depending how much newer it is, you might find the running config slightly different from the saved config. Next time you save the running config, the startup and running configs should match. However, if for any reason you need to regress your IOS version, the older IOS might not recognize all the newer IOS syntax. So, you might want to save a copy of your older config for a little bit.
