If you are using a UML tool integrated with an IDE, working alone, and not doing wall sketching, then synchronizing the diagrams is a simple reverse-engineering operation in the IDE.
But what if you are working with a small team and want to spend a modeling day each iteration at the whiteboards, applying UML as sketch. Consider this scenario:
At the start of a three-week timeboxed iteration, there was a modeling day involving UML wall sketches.
This is followed by about three weeks of code and test.
Finally, it's time to start the next iteration's modeling day.
At this point, if you wanted to do some wall sketching again, based on the existing state of the code base, how to proceed? Here's a suggestion: Just before the modeling day, use a UML tool to reverse engineer the code into UML diagramspackage, class, and interaction diagrams. Then, for the most interesting ones, print them large on long plotter paper, on a plotter. Hang them relatively high in the modeling room on the walls, so that during the modeling day developers can refer to them, sketch on top of them, and sketch below them on whiteboards or static cling sheets.