Yes, if science ever gets to the point where it can replace memories, I think that'll be a positive. You could use it to rid a child of the memory of molestation, for instance. I feel as if that would certainly do wonders for a child as they grew up. It would be revolutionary.
Yes, certain memories should be replaced in order to have a richer life experience, because there is no reason to suffer painful memories if it can be avoided. The memories might be something that holds a person back, if they were traumatized. It would be good to not force the person to continue to think about them.
What if it turned out that crafting a set of perfect memories and implanting them into a material brain caused some type of terrible imbalance in the emotions or personality of that individual? What if it turns out that we require darkness and pain as part of our life experiences in order to give us those higher qualities that we ascribe as "human"? Would an individual who had this done to them even really act/speak like a real person any longer or simply a gleeful robot? It seems like it's too chancy to me.
I don't believe in fate, or destiny. But I do believe that in order to move forward, to advance, to progress, we need the complete range of human experience to exist. What I mean is, every person is a cog in the machine that makes the world move. Some people have lived far worse lives than others. Some people have experienced terrible things. However, we need those people and those experiences to build upon for the future. People can never improve if they are never dissatisfied with the way things are, and people will never be dissatisfied if they don't think anything bad has ever happened. This idea, which has probably been around for a long time, but was most famously addressed in Eternal Sunshine of the Spotless mind, always makes me think of Brave New World by Aldous Huxley. The people in it take a drug that makes them happy no matter what, and because of that society has come to a total halt. If there is no misery in the world, there can be no joy. Without a reason to improve things, nobody would feel the need to invent. Replacing negative memories with positive would make people forget that things CAN be bad, and so they would have no reason to try and improve. We need a broad range of experiences to think of new ideas, and you can't get that if everybody is happy all the time.
I do not believe it would be mentally healthy for humans to alter their memories. I believe our life has purpose and we shouldn't change what we have experienced because it is possible we had those experiences for a reason. Removing certain memories would alter who we are and I don't think much can be gained from such actions.