Unaccelerate

A class of claims or interests is impaired unless the plan does one of three things with respect to the claims or interests contained in the class?' “leaves unaltered the legal, equitable, and contractual rights” of the holders of claims or interests in the class; unaccelerates any acceleration by curing defaults, reinstating the original maturity of the claims contained in the class, compensating for any damages suffered as a result of the default, and not otherwise altering the claimants' legal, equitable or contractual rights.

Can the bankruptcy judge undo this and "unaccelerate" the mortgage debt?

Once a loan is accelerated, it's almost impossible to get the lender to "unaccelerate" and reinstate your old loan.