So... As I understand this; You have a bunch of files in reverse order & want to rename them in proper order? So that 1, 2, 3, 4 becomes 50, 49, 47, 46 (or vice-versa)?
The solution to that is simple: On the batch rename dialog, click the "sort by" drop down and set it to "name (descending)". This will reverse the order of the numbering.
^ Note how the order of the original files is reversed, while the order of the new names remains the same, thus renumbering the files in reverse order.