Is it permissible for someone who is accustomed to giving a monthly charity from his salary to change his intention so that it is on behalf of his parents and grandparents, those deceased and those living, with the same amount remaining unchanged?

There is no harm in the giver of charity dedicating the reward of his charity, which he spent from his own money, to whomever he wishes. This is not a deception against Allah. However, it is more befitting for him to keep the reward for himself, out of a desire to perpetuate goodness, because the most beloved deeds to Allah are those that are most enduring. As for dedicating the reward of charity to the deceased, it is permissible without dispute, although it is more befitting for him to keep the reward of his charity for himself and increase his supplications and seeking forgiveness for his deceased.