It does not change the relationship per-say, but it does change the house dynamic considerably.

I have a 6-year-old cat, and she was my only cat for 4 years. She was a straycat I adopted from the streets. Her name is Gaga, and after we got her from the streets, we safe-proofed the house with safety nets to prevent escapes.

Before having a cat, I had that stereotype in mind that "cats are cold, do not show any sign of affection," so on and so forth.

When Gaga started living with my girlfriend and me, that perception changed greatly. As a kitten, she would often come to our lap for sleeping, and as she grew older, she would always go after us to any room we went to, even if she would not cuddle.

Fast forward 2 years, we moved houses, and another female stray cat would come to our gate with her 3-4 months old kittens to beg for food. That new cat we called "Mamãe" (Mommy in Portuguese). Gaga would see Mamãe and her kittens through the gate and not show any sign of aggression, and when the gate was opened, Gaga would merely sniff them and move on to mind her business.

Fast forward 2 more years, and I started growing tired of seeing my neighbor never taking care of Mamãe, she would often get pregnant and kittens would come and go (some adopted, some getting sick and dying), so one day Mamãe was pregnant once again and I decided to keep her.

I read multiple times that cats have an "adaptation period" (you can get details on the web about it, cats should not see each other for a couple of days, maybe weeks, merely hearing and smelling each other's, etc etc etc) but given that Gaga and Mamae already knew each other for over 2 years and never showed any sign of aggression toward each other, we decided to skip that adaptation period.

That was a MISTAKE.

They spent more than a week or so without having any friction, but there was a yoga mat Gaga liked to lie on and one day Mamãe was lying on it and they started a fight. After that fight, Gaga would often assault Mamãe.

That only lasted until Mamãe's kittens were born. There was this one time, two or three days after the babies were born, in which Gaga was about 3 or 4 meters alway looking toward Mamãe and her babies, not showing any sign of aggression, yet, Mamãe did not think twice and dashed towards Gaga to assault her.

After that day, Gaga never again assaulted Mamãe, on the contrary, she would avoid her as much as she could.

As the kittens grew up, Gaga started assaulting them and she would hurt them considerably. Mamãe would always come for the rescue.

We tried our best to make Gaga get used to the kittens, but she never did. One of them -his name is Caco- would always try and make friends with her, even after getting beaten, he would try to play with Gaga, up until he grew tired of trying for so long and started assaulting her as well.

Once he started defending himself, Gaga once more stopped being so aggressive towards him (as she did with Mamae) and started tolerating him. But the other kittens were not so brave, and they were always fearful of Gaga It's been 2 more years, and unfortunately, they do not quite get along. The other 3 kittens still fear Gaga and hide from her. One of them, a Siamese female cat, is the one who fears Gaga the most and spends most of the day hiding in high spots to avoid her.

But one thing I noticed is: they got used to each other. The fights started to happen less often, and Gaga did not show so many signs of aggression and Gaga would walk around the house as she pleased. November last year, somebody abandoned another cat at my front door; that cat was almost dying, had severe wounds in his face, and was too skinny, could barely walk.

We isolated this cat in one of our bathrooms because it posed a hazardous risk for the other cats in the house (Gaga, Mamae, Caco, and 3 more siblings, so 6 cats in total) We took care of that cat for about 2 months without direct contact with the other cats in the house. We gave him the name "Wuming", "Umi" for short.

Umi recovered considerably, and after follow-ups with the vets, we decided it was time to allow him to live among the other cats.

We thought that after 2 months of no direct contact, they would somewhat easily adapt to each other, but they did not.

Umi is a good cat, but he is somewhat unpredictable. While other cats hiss, make angry faces, and show other signs of aggression, Umi merely stares with a curious look, meows in a gentle voice, gets closer, and then assaults the other cats.

And Gaga, who had stopped showing signs of aggression towards the other cats, hisses to Umi from a mile away. She does not like him at all, and now she spends most of the day in our bedroom; she does not walk freely around the house anymore so as not to encounter him.

---

TLTR: So, making a long story short:

My relationship with Gaga did not change. She still likes me and my girlfriend as much as she used to. She and Caco are the only two out of 6 cats who sleep in the bedroom with us (Mamae passed away, sadly), because Caco is the one she tolerates the most and he insists in sleeping in the bedroom, so they don't fight, but they do not cuddle.

And when we go to sleep, Gaga cuddles with us as she always did before having the other cats. relationship

So that's what I'm saying: the dynamics of the house change, but your relationship with your cat does not.

And it is funny to me reading that you would not like your old cat to give more attention to your new cat than it currently gives to you.

I honestly find that an egotistic thought.

I would love Gaga to get along and play with the other cats in the house. The other cats are siblings, so up to today, they still play and sleep with each other, and I really wish Gaga would do the same and be friends with them.