What actually determines whether it is possible
Most cleaners work more than one site in an evening, so moving one attendance moves the others. A two-hour delay at the first site can push the last site past the point where the building is accessible. That is invisible to the client asking, and it is the most common reason a request that sounds trivial cannot be accommodated on the day it is made.
The second constraint is the person. A cleaner rostered to finish at nine may have arrangements that make a later finish impossible, and it is not reasonable to require it at a few hours' notice. Where an alternative person can cover, that works, provided they know the site. Sending an unfamiliar cleaner to accommodate a timing change usually trades one problem for a worse one.
