How should I go about re-releasing a GPL-licensed project?

Question

A few months ago we found a library licensed under GPL that fit the bill for what we were looking to do at that time. We included it in our codebase and all was fine. Now, a few frantic months of coding later, we've refactored the hell out of the library: it's more feature complete, more stable, fully unit tested, PSR-0 compatible, etc.

Now we would like to use the library in another one of our projects and that got me thinking, why not re-release the library?

The problem is that I don't have any idea on how to attribute the work the original developers have put in (which is actually that much refactored that it's kinda unrecognizable) when releasing the library as GPL again. Over time all the file documentation headers with the original credits have been replaced and all that remains is the LICENSE file which is an exact copy of the GPL v3 license.

I have absolutely no problem giving credit where it's due, but I would like to do it according to what is right in the FOSS world. Anyone can tell me how to proceed?