Is it reprehensible for a mother to go back on her decision to wean her two-year-old daughter and breastfeed her again until she completes two years of nursing, given her ability to do so?

There is no harm in weaning an infant before two years if it does not cause harm, and there is no harm in extending beyond them. As long as the mother is able to complete the two-year period, there is no harm for her in reversing the decision to wean, and there is no dislike for that; rather, it may be obligatory if the daughter needs it.