I'm sorry to be the bringer of bad news, but that would be near impossible since the sig is a .jpg
If you can contact the original person who made that sig and he still has a photoshop version saved, then it is very much possible. But if he hasn't, I'm afraid you'd either have to ask for a completely new sig or stick with what you've got.
meshawn is right bro...unless you have the photoshop file you cant change it if you cant track it down and want a sig similiar to yours with those guys....just hit me up in my thread a little down this page called "my first sig attempt" and ill see what i can do...or another guy in here can do it also. good luck
As I've said through pms, I forgot to save the psd file. I could make you a new one, but don't expect it until the weekend. If you would like one now, try asking another sig designer. They should be able to help you out.