I am a US citizen, had applied for my parents GC in October 2009. Mom underwent fingerprinting three times here, but did not get her AOS done in time. End of last year my father (who got his EAD and AOS smoothly) had emergency heary bypass here, and then they had to leave back for India for more tests. Mom left on AP. Subsequently I wrote a letter to NBC, MO to ask them to expedite my mothers case. Yesterday I received a letter stating that she has go to Philadelphia USCIS office next week for "Completion of application to register for Perm residence." Mom is in India now.
You need to contact USCIS customer service for guidance. Definitely put something down in writing that you are requesting a rescheduling. Maybe asking your Congressman's assistance is the best way to go for rescheduling.