Is the prayer of one who intended to say a specific dhikr after rising from rukūʿ, or to recite specific sūrahs during the prayer, valid if they then changed their intention during the prayer to say another dhikr or to recite other sūrahs, whether intentionally or out of forgetfulness?

A mere intention to recite a specific surah or utter a particular dhikr during prayer does not make it obligatory; the worshipper may switch to something else. However, if one vows to recite a specific surah or part of it, they must fulfill what they vowed, and nothing else will suffice, even if it is longer. This is analogous to someone who vows to give charity in silver but gives gold instead; it will not suffice.