Ranaming a (mostly) numbered series of folders

Hey everyone! Quick question! I searched these forums but couldn't find an answer...

I started off with a series of a hundred folders, all numbered and titled like this:

001 - Yellow
002 - Blue
003 - Red
004 - Black

Then, I've added more folders to the list, in between the original folders, adding an "a" where I want the folders to fit. Now the directory looks like this:

001 - Yellow
001a - Green
002 - Blue
002a - Purple
003 - Red
004 - Black

Basically, is there any way I can automatically renumber the folders so that the result is this:

001 - Yellow
002 - Green
003 - Blue
004 - Purple
005 - Red
006 - ...


Thanks for your help in advance!

Edit: I figured it out with a little Google-Fu. Using <Substr:6:30> , I'm able to keep everything lined up.

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