In the past when I've had this same situation where the GL numbers were the same, but not, I had to renumber in two steps. I first renumbered the GL into a completely different set of numbers (this is to free up the once used accounts) and then perform a second renumber to move them back to their new accounts.
Since you are using 8 characters and have a free space, you might consider expanding the third segment to 3 and adding a 9 to all accounts. For example: 1000-10-10 becomes 1000-10-109. Once you finish your last renumber you can reduce the account back to 8 characters. Sorry, I can't remember in 3.71, but may to reduce the account structure back to 8 through DFDM. As long as the last character is zero it should not matter.
Absolutely do this in 3.71 because it considers the entire GL account number as unique so you wont need to deal with segment validation rules, etc.