HBM-I would say a 5K yea, but a 10K might be pushing it? How "bad" are your old shoes-if they aren't that bad, I would run in them. If they are giving you pains-I would take the new blisters which you would most likely get from racing in a new pair-over a potential injury.
Well, my knees have been hurting and the bottoms of my feet are sore with the old shoes. But blisters sure sound painful from new shoes. I guess I'll go with the old ones. Does anyone take ibuprofen before a run to help ward off pains that might develop during the race? I think I might try that.