Not necessarily. Apple could simply delete all code modified to make that change, necessitating a similar amount of work for each phone unlocked.

Apple has said that for legal reasons, it may be forced to keep the code permanently and will have to secure it permanently out of concern for future legal/court obligations specific to this case.

In support of this, someone forwarded me a very interesting article written by someone who creates forensic software for a living. The legal requirements surrounding the creation of a software tool for forensic purposes, which this proposed effort requested by the government might fall under, are nothing less than herculean in scope.

http://www.zdziarski.com/blog/?p=5645

Zdziarski's arguments are very illuminating on how this is not a simple or one-off request. Excellent read.

I'm sure defense counsel would want to be able to verify that it isn't modifying file access times, or deleting data, or planting data, or otherwise disturbing evidence when the update is put in.

If Apple had designed the iPhone to require user authentication before updating the software/firmware then they wouldn't be in this mess and they would not be able to comply with the court order short of hacking/jailbreaking the phone. If the pin was required to be entered before installing new software on the phone then the FBI would first need to know the pin to load GovOS on the phone so they would not be able to crack the pin using this method. And once Apple patches their software to require user authentication before installing updates they will no longer be able to comply with any similar type of request.