I think you may be trying to hold onto grades you don't need anymore. If your course total is going to be the 40% from the original assignments and 60% from the new exam, why not change the original exam to 0% of total and new exam to 60%? You don't have to delete the old exam from the gradebook, just change the weighting so it doesn't affect the course total. I think it would be really confusing to users if you tried to get two totals right next to each other in the gradebook.
Happy 
oodling!