Is it permissible to convert the orphan's sponsorship fund into a donation for cancer patients, despite the difference in reward for each, after it became impossible to send the fund to the orphan?

It is preferable to appoint someone other than that woman to ensure the money is spent on sponsoring an orphan, or directed to the sick, as both are good deeds for which the doer is rewarded. "And whatever you spend of anything, He will replace it; and He is the best of providers." The knowledge of reward is with Allah, but Allah Almighty recompenses whoever is sincere and seeks good.