The kitties were nestled all snug in their bed....

[Glamrock]

8. My experience

My cats always hate the new one. For a few weeks. Eventually, they accept it and make friends, or just accept it. The stress goes away. I'll caution that at 9, it may take longer than a few weeks. Maybe 4 or 5. But I doubt it. If they seem to be curious and friendly through the screen door, they'll probably be okay. Perhaps bring yours out with you on the porch a few times and let them get to know each other.

I'm not a fan of free range cat ownership (i ran one over tonite. Broke my heart) I'd bring in the other. If that is what you ultimately decide, you'll need to take it to the vet to be sure it doesn't have any disease that could be transferred to yours. Also, if yours is declawed, you'll have to do the same to the other, in my opinion. If you don't, you are risking injury to yours and a lifetime of fear and stress.

All that being said, I'm no expert. Nor am I a vet. But that's the best advice i can give based on my experiences with 7 in my lifetime. Hope it helps. And I hope you'll keep me informed!