This very simple application really sorted me out after I had modified my short name using Apple's migration assistant ( in the process of a backup before my computer went for repairs). As you will learn from reading the very usefull warning notes provided with this program, you should NEVER change your shortname unless you really have to. And I am still dumbfounded to understand why Apple's own migration assistant program lets you do it with no warning ...
At any rate, if you happen to end up with a changed short name after migrating, a lot of useful things will stop working, like the automatic opening of your keychain, or the arborescence of certain applications like dreamweaver. And this fabulously simple program allowed me to revert to my old shortname, and retrieve all these functions.
All I have to say is THANK YOU VERY VERY MUCH to the developpers who were constructively very reactive to my queries ( you will soon find 10 $ on your paypal account ;-)