I just spent 30 mins trying to find a research article on spaying and aggression. I did find one on rats, but the one I had before was dogs.
http://www.associationofanimalbehaviorprofessionals.com/dog.html
My head hurts, I give up. So let me state I am old and senility is in question, but basically this is what I recall.
In some rare cases, bitches spayed have exhibited increased aggression and needed hormones to help. I will write Wendy and ask since I am fairly certain she was the one who pointed me to it years ago (She owns Mar Vista Animal Hospital and is the editor for the Veterinary Information Network). At any rate, this was with bitches who already demonstrated aggression and evidently removing the hormones that helped calm increased it. But it is VERY rare.
In general, spaying shouldn't change your dog at that age. There are some studies on pediatric spay/neutering where the brains and body never get the hormones, but that also isn't your situation.