Depends in part on the popularity of the restaurant along with all the reasons mentioned above.
I wouldn't try upping the count at places like Ohana, BOG, etc w/o having an ADR w/ the booked head count.
9 Dragons is rarely full, Kona is flexible but might require a beeper wait, most hybrid QS eateries that take ADRs can accommodate.
You might be able to make a reservation at many Disney Springs places with ease and and cancel your existing ADR.
What restaurants are you concerned about?