Overclocking within reason isn't going to kill your processor. Yes it may not last as long but it will, in most cases, be well outdated before it fails. I've had processors running overclocked for years that get used nearly 8 hours a day and they're still living. Examples are my 2500k oced to 4.5 since 2012, my Fx 8350 oced to 4.7 since 2012 and my 955 Be at 4.1 since late 2011, all are still running strong.
Yes you can overclock without changing the voltage per se. but if the voltage is left on auto the motherboard will increase the voltage to be able to cope with overclock, to a point. So if you were interested in seeing how far you could push a cpu on "stock" voltage you would need to set it manually to it's stock VID, then Oc.