No, you cannot rename keys once that have been added to a Dictionary. If you want a rename facility, perhaps add your own extension method:
public static void RenameKey<TKey, TValue>(this IDictionary<TKey, TValue> dic,
TKey fromKey, TKey toKey)
{
TValue value = dic[fromKey];
dic.Remove(fromKey);
dic[toKey] = value;
}