Is it possible to change the intention of completing a recitation of the Qur’an for someone after having verbally expressed it, and then to retract and change it for someone else?

Donating the reward of a voluntary act of worship to others is permissible. If you have already dedicated it to one person and informed them of that, you cannot redirect it to another. However, the opportunity remains for you to perform other recitations (of the Quran) whose reward, or part of it, can be dedicated to your mother.