The group over at NIL have written an nice article on the configure replace command. I've seen this command a couple times, but never used it. It allows you to replace the running configuration with a copy of the startup configuration (or some other config you have on a TFTP server). Previously, this was only possible by rebooting the router or using a ton of "no" commands to back all your configuration out. The router can even list what commands will be negated as it replaces the running configuration! Nice!
The command to pull it off is this:
Router#configure replace nvram:startup-config list
The "list" command on the end will list the syntax the router is adding or removing as it goes...very nice! The entire write-up on this can be found here.