Yes, you can.

Your memory of that person is just that- your memory. You can re-write it to involve a different version of that person, or you can write that person completely out of the memory- it doesn't matter. That person is completely different right now from the person you knew, and your experience of that person is completely different from everyone else's. So you're not changing something static- you're just putting together new memories and connecting with others in a different way. Don't limit yourself to a false sense of "realism"- the only realistic worldview is the one where everything is possible. Take that as far as you can, and only then will you know which limits are the ones you must have chosen for a reason.

What matters is to re-write that person in a way that is most believable to you. So you can use your creativity to create a positive version of the memory of that person that seems write to you- most likely because it would make the person you remember happy. But that's the person in your memory- you don't have to worry about all the other similar connected people, it's you you're working on- but it's still good to be as kind and generous to your co-creative partners in your imagination, because it will make you feel good, and then the positive effect on people in your life now will come automatically. That's transcending consideration, where you really have mutual goodness.