Do you know that cat behavior is directly affected by the environment your cat lives in?  
That’s right. Cats are not indifferent to their environment, in fact they love stability and any social change may have an impact on the way they behave. Just like most pets, your cat can adapt but it will take time.

If you plan to introduce a new cat to your old one, there is a possibility that your old cat may distance herself from the new cat. She may react by hiding for several days or she may act up to make the newcomer retreat. Don’t panic. There are ways that can make the transition easier for your two cats.

In case your new cat becomes threatened, you should give her an area where she can retreat by herself if you have a spare room for your new cat that would be better. Why do you need to do this? By placing your cat in a spare room with the door closed for two to three days will help the two cats to adjust to each other’s scents and with the door closed you prevent your pets from hurting each other. Separating your two cats initially should give them the time for some adjustments thereby improving cats’ behavior.