These 2 are young, as I recall -- 9 months! So start now with their new names. You can hyphen the names for a week or so, so they get used to hearing it and know which of them you mean.

When Alicat came here, I changed her name to Mandy. I said "Ali= Mandy" for a while, then Mandy Cat for a while. Once she would look at me when I said her name,* I moved on to the next level. Now, she is just Mandy.

*Keeping in mind that these are CATS of course; they don't always WANT to acknowledge you, lol! I always had treats in my hand to help with that.