Could an employer change the terms of an outstanding loan - in this case, if the policy in place at the time the participant took the loan utilized the maximum permissible cure period could the employer change the policy so that the cure period for that loan was shorter than what was communicated to the participant at the time he took the loan?

I don't think so. My guess is that the participant's cure period was stated in the promissory note which is what they signed and agreed to and that can't be changed.