Our C-corp made a business loan to a LLC. The LLC had difficulties paying the loan and wants to redo the loan without interest.

Legally, I believe that yes, your C-corp can forgive the interest receivable and renegotiate the terms of the existing debt. However, from a tax perspective (meaning can you deduct the bad debt on the business tax return) if the LLC is a related party, then you may not be able to take a tax deduction. Additional information would be required.