...should be the ultimate goal. Understand the design, get it under basic control and then work with a team (largest you can muster) of diligent specialists to design replacement systems that are firewalled off from the original. The reasons for this are twofold:
1) No matter how well documented, well designed, etc. the system is, your knowledge of it will never be perfectly complete and you'll never be able to turn around changes with the same degree of confidence and alacrity as the original admin.
2) Your Career -- If you bend over backwards to make the system work perfectly the original designer will still get most of the credit. If you try your best but the system falls short of expectations, you will take the blame as the new "owner" of the system. It's a lose-lose proposition. Building something new, something that you can demonstrate is supported by more than just one person (unlike the original) will be a feather in your cap.